In Reply to: Re: Synthetic oil rules! 10W30 Mobil 1 ???? posted by Jim on March 19, 2000 at 13:43:14:
The "40" controls its viscosity at operating temperature. The "10W" means it is a little thinner when very cold (compared to 15W). This is usually considered advantageous since the oil can circulate faster on initial startup, even when very cold.
In the "old" days, having a wide viscosity range, cold to hot, required compromises. For this reason, many knowledgeable people would use "straight 30" weight, not multivis 10W-30.
Today, the synthetics are so good this is not a consideration.
Of course, if you feel safer using BMW oil, I'm sure that's perfectly fine. Most people prefer Mobil 1, however.