I use 10-30 mobil full syn in my e38 but recently topped off with a half qt
0-40 mobil full syn (for newer euro cars). Could this cause a problem or will it be ok. A mechanic told me to always use same weight but it wasn't available.

yes, you should always try and use same weight oil. that said, it is worse to run your engine low of oil than it is to run different weights. just make sure use all the same weight when you change the oil again.

Should not be a problem. I am surprised you could not find the 10-30 since it's the most common version. BTW around here you can buy the 5-quart jug at Walmart for about $20 IMO a bargain.

If it bugs you, look at the Mobil-1 website, call their tech line, or just go ahead and do your next oil change early.

To avoid the situation in future put a spare quart or 2 in the trunk...