I have seen this on a couple 3.0 engines including my X5. On my X5 it was more Green than Yellow. It was primarily when it was cold and it would collect on the bottom of the oil cap. I never did get to the bottom of what it was, but I suspected water. After pressure testing my cooling system and compression to eliminate a serious problem I did a couple other things and it has not returned in about 20k. The first was to replace the oil cap, the seal was worn and moisture could enter there.

The second was to check the secondary air pump for moisture. It’s located to the left of your engine a plastic tank with a hose going to a valve on the exhaust manifold. There is a vacuum line on that valve and if the vacuum line is broken the valve will let moisture into the EGR system and that has the potential to introduce moisture.

Finally I change the oil to BMW LL04. The only oil available in the USA that I can find that is BMW approved with that spec is Elf. I have not seen this since I did those things. I can’t tell you if they solved the problem or the problem naturally went away.

Good Luck.