Differential Oil Efficiency **DATA**

[Graham E39 528i]

Dawgzz,

Smallsnails on this forum said switching to Redline 75w90 made quiet his noisy diff.

BMW4te said his 530i was 'slowed down' by the Redline (over on the site that rhymes with incest):
"....but the most annoying thing was that it slowed my car down ! Before the change I've always picked up speed when going downhill, even on slight down slopes. Well, not anymore after the change . And before it was only needed to push the gas pedal every now and then when going straight on a flat road in order to maintain a constant speed. After the change, I had to push the pedal all the time as I could feel the car "braking" the moment I lifted my foot from the gas. It was like somebody was continuously pressing the brakes !!! Then I realized I had the same thing happening to me on my previous 325 after changing to same RedLine 75W-90. RedLine is thicker than OEM Castrol SAF-XO which I think partially explains the results."

Later he gives some interesting data:

"Based on what I've read SAE viscosity ratings for gear oils do not have the same relevance/significance as with motor oil.

In my opinion the thickness of the gear oil is only half of the story (but at least it is documented). The other half is the friction additives added which follow a proprietary formula. I believe the only way you can tell whether those additives will work with your differential (when not using OEM oil), is to test the product.

Below are some viscosity numbers that are publicly available:

Castrol SAF-XO (OEM Factory Fill)
=====================
Viscosity 40C: 100 cSt
Viscosity 100C: 15.2 cSt
Viscosity Index: 160

Castrol Syntrax 75W90
===============
Viscosity 40C: 117 cSt
Viscosity 100C: 15 cSt
Viscosity Index: 139

RedLine Gear Oil 75W90
===============
Viscosity 40C: 115 cSt
Viscosity 100C: 16.4 cSt
Viscosity Index: 155

Castrol Syntrax Long Life 75W90 (OEM replacement)
================================
Viscosity 40C: 103 cSt
Viscosity 100C: 15.7 cSt
Viscosity Index: 162

Mobil 1 Synthetic Gear Lubricant LS 75W-90
===========================
Viscosity 40C: 99 cSt
Viscosity 100C: 15.2 cSt
Viscosity Index: 142

AMSOIL Severe Gear Lubricant 75W-90
=========================
Viscosity 40C: 109 cSt
Viscosity 100C: 16.8 cSt
Viscosity Index: 167

Motul Gear 300 75W-90
===============
Viscosity 40C: 72.6 cSt
Viscosity 100C: 15.2 cSt
Viscosity Index: 222

Hope this helps"

Very interesting data. Amsoil AND Redline don't do very well in this survey.

I'm using Mobil 1 now, which at 100C is about as good as I can get from an efficiency standpoint. Castrol Syntrax does a little better. At 40C the advantage goes to the Motul oil.

I need to measure the temperature of my differential, obviously.

PS A big thank you to BMW4te

[DHoang]

as much as i want to believe this....but you mean to say that this guy BMW4te can feel how 1.5qt of gear oil is able to slow down a 3500lb car rolling down the road? For real????????

[rf900rkw]

+1. The drag to do such I would expect to be enough to cause a significant increase in diff temp. I may have to put this as the next data collection thing after I finish my ATF tests.

[Graham E39 528i]

as much as i want to believe this....but you mean to say that this guy BMW4te can feel how 1.5qt of gear oil is able to slow down a 3500lb car rolling down the road? For real????????

Your scepticism is healthy. I'm sceptical too.

Suppose a really good diff had a very thin oil, that caused the efficiency to be 99%. The 530i puts out ~240hp, so if 1% of the energy is left as heat in the diff, that is 2.4hp as heat.

Suppose the same diff with a very thick oil were used and the efficiency had dropped to 96%. The heat build up in the diff would be 9.6hp.

My example is extreme, but would one feel the loss of (9.6 - 2.4)hp = 7.2hp?

Probably so.

[topaz540i]

The force required for the diff oil to slow the car would require a viscosity beyond road tar.

But plus one on the mobil1. Im about to use that in my m5 rear after i get a chance to take it apart to examine the clutches.

[JimLev]

The force required for the diff oil to slow the car would require a viscosity beyond road tar.

But plus one on the mobil1. Im about to use that in my m5 rear after i get a chance to take it apart to examine the clutches.

So what's the weight of the M1 your going to use? The OEM fill for the M5 diff is 75w/140 which is closer to road tar. Maybe that's why the rear cover has cooling fins on it.

[topaz540i]

The rear cover has fins to cool the burning asphalt off. And because it looks cool.

Im using 75w140. Why does 140 have to cost exactly twice as much as 75w90?

[Graham E39 528i]

The problem, of course, is knowing the viscosity doesn't tell you actual efficiency of the differential. A good experimental method would be to choose the oil that causes the outside of the diff casing to be heated up the least.

I think I will try the Motul 300 75w-90. It leads the pack in low temperature viscosity. It's more expensive than Royal Purple.

[JimLev]

What are you paying? Redline 75w/140 cost me $10 a quart. Small amount for the ability to burn up both tires evenly.

[Graham E39 528i]

+1. The drag to do such I would expect to be enough to cause a significant increase in diff temp. I may have to put this as the next data collection thing after I finish my ATF tests.

What are you gonna test? I'm interested.

I'm sick of reading threads about oils where some guy says:

"I've been using Royal Purple for ten years with no issues"

and I feel like saying:

"The reason you have no issues, dude, is because you have no metrics, DUDE!

[cnn]

topaz,

75W140 is mostly used in semi trucks pulling 30,000 lbs of cargo etc.

I have been using Redline 75W90 now for 6 years.
I drained the factory fluid and replaced with Redline 75W90 in May 2006.
I didn't notice any change.

[Cyrix2k]

I think the claim is being exaggerated but I wouldn't be surprised if a difference could be felt. Using 240hp as a baseline for cruising is fallacy; that's at peak output. Cruising is going to result in a low number. A quick google search reveals that the efficiency of a differential is in the region of 98-99% so I suppose they do generate a reasonable amount of heat. I have never checked. As an aside, I too run Mobil 1 synthetic gear oil.

Now for actual power numbers, I am far too lazy to even attempt to work out cruise power from a mathematical standpoint. However, if these guys are even close to accurate (and I'd wager they are), it takes well under 20 hp to maintain a cruising speed of 50 mph in an insight - I don't think our cars have that much more drag. Do I think the diff oil could have a noticeable effect on something that requires such little output? In a word, Yes. Can you feel dragging brakes, even light dragging? Yup. How about deflated tires? I can. I see no reason this isn't plausible.

Going back to power output again, it is easy to see why the M5 has a finned diff cover - that is a car built to cruise at high speeds which results in a lot of power being transmitted to the wheels and thus higher losses (= heat) in the diff.

[James39]

topaz,

75W140 is mostly used in semi trucks pulling 30,000 lbs of cargo etc.

I have been using Redline 75W90 now for 6 years.
I drained the factory fluid and replaced with Redline 75W90 in May 2006.
I didn't notice any change.

M5 calls for 75w140 in the diff ... for that reason, I'll put it in my m5 diff too.

http://www.m5board.com/vbulletin/e39...-vs-75w90.html

[pleiades]

Nothing worthwhile to add here but ....

Lowest measured diff. surface temp = highest "efficiency"? What sort of temp gradient are you expecting or observing across these different oil brands/grades? Don't some of these grades work better (i.e., protecting the internals, as coolant and lubricant) once they're up to a certain temp? If the diff is causing extra drag with oil X, could we expect it to make more noise? Would coasting downhill in a specific gear reveal drag from the diff or more likely the trans?

I'm subscribed, hope to learn something about this from you guys....

I have Redline 75w90 in both my cars (e39 and Subaru with two diffs). Haven't noticed any drag since changing to Redline in either.

[TriniSpeC]

notice, the reason you might feel the fluid slow the car down is because the old stuff you drained out was water like with zero viscosity.

[mifesto]

i concur. my rear diff oil was nasty at 120k miles, probably original. redline went in. it did feel smoother and buttery but never felt drag.

[Justin517]

The rear cover has fins to cool the burning asphalt off. And because it looks cool.

Im using 75w140. Why does 140 have to cost exactly twice as much as 75w90?

why are you running 75w140 in an open diff? they call for 75w90.

I agree though... mobil 1 and Royal Purple 75w140 both cost $20/qt. while 75w90 is $10/qt... doesn't make much sense to me.

[Graham E39 528i]

Lowest measured diff. surface temp = highest "efficiency"? What sort of temp gradient are you expecting or observing across these different oil brands/grades? Don't some of these grades work better (i.e., protecting the internals, as coolant and lubricant) once they're up to a certain temp? If the diff is causing extra drag with oil X, could we expect it to make more noise? Would coasting downhill in a specific gear reveal drag from the diff or more likely the trans?

I'm subscribed, hope to learn something about this from you guys....

I have Redline 75w90 in both my cars (e39 and Subaru with two diffs). Haven't noticed any drag since changing to Redline in either.

Pleiades,

This whole thread started because we have three pieces of data.

• SmallSnails reported the Redline 75w90 made his diff quiet.
• BMW4te reported the Redline slowing his 530i
• BMW4te reported the Redline slowing his 325i

Three pieces of data, not much. If we accept the data, then the data are self-consistent. Why? Well, we know why the Redline made the diff quiet. It is so gooey, it damps down the chattering of the differential AND slows the car. We need more data obviously, though.

Your summation of the surface temperature of the diff is correct. If we ignore all other effects (like anti-corrosion, gear surface wear, limited slip ability, et cetera) we should be using the oil that delivers the lowest differential casing temperature.

There is one easy experiment we can all do. Go for a drive until the car is warmed up, then check the temperature of the casing with a finger. 40C is bathtub temperature. 100C is a boiling kettle. Which is closest to the truth?

I have a friend with three Porsche Turbos. He has a 'temperature gun', and he is going to measure the temperature of his diffs.

I agree though... mobil 1 and Royal Purple 75w140 both cost $20/qt. while 75w90 is $10/qt... doesn't make much sense to me.

Justin,

If you knew of an oil that would give you more than, say, 1.5hp at the wheels than other oils, wouldn't you pay $50 for it? Maybe more?

[Jynger]

by reading this thread I remembered that my old 1996 e39 520ia went from 9l/100km to 12l/100km in fuel burning after diff oil change. Diagnostic by bmw and by many different specialists never shown any errors, everything was in top condidtion, just the fuel consuprion mystery remained. Diff was never separately checked, maybe it was broken in oil change somehow, dont know, didnt even think to search for problem there.It was 5 years ago and the 12l/100km never went down again, sold it.

[Graham E39 528i]

by reading this thread I remembered that my old 1996 e39 520ia went from 9l/100km to 12l/100km in fuel burning after diff oil change. Diagnostic by bmw and by many different specialists never shown any errors, everything was in top condidtion, just the fuel consuprion mystery remained. Diff was never separately checked, maybe it was broken in oil change somehow, dont know, didnt even think to search for problem there.It was 5 years ago and the 12l/100km never went down again, sold it.

Interesting. Which diff oil caused the increased fuel consumption?

[Jynger]

all I remember, I got some mobil1 oil prepared for the change, but when I got car back, the guy said he got some "better and more expencive oil" that he filled it with. Now reading, more expencive = beyond road tar viscosity . oh and, I think it was purple or red in color, thought it strange as I had never seen that color oil/grease before (the droplets could be from dirty cloth too).

[pleiades]

bmw4te may be on to something and I'm interested in finding out, but to nitpick, in the thread where he pointed the finger at Redline as a cause of the downhill drag he says he experienced in his e39, he neglected to mention that he had just done a triple fluid change (engine, trans, diff).

MPG drop after triple fluid change ...

[Graham E39 528i]

I didn't see that. The thread I saw is on chapter 3 of:

http://www.bimmerfest.com/forums/sho...d.php?t=460736

I tried PMing him a couple of days ago, but no reply yet.

[Graham E39 528i]

I strapped a thermocouple* to my differential this morning. After a 28 mile trip on the freeway cruising at about 65 mph, it got to 56.5 C. Tip of thermocouple inserted into a deep depression on the casing (below the fill plug and filled with ear plug wax). Ambient temperature was 53 F (~12 C).

Given how cool this is, the viscosities at 40 C on the OP data are a better guide to where my oil choices will be thermodynamically (or better, a linear interpolation between the data at 40 C and 100 C).

Strength & Honor



*attached to a brand new Fluke 87 (Mk V)