Final Drive Oil Change?

[bmwrich58]

Lads, just to clarify the oil spec in the final drive am I correct in saying that the SAF XO is a castrol product and it is actually a semi synthetic 75W/90 , is it critical to use the castrol version.

Here's what the US Dealers are using and what I'm using...

after 10K miles and a trip to Alaska the oil was still red and clean but the drain plug had this in it.

after cleaning you can see the magnet