In Reply to: Re: Oil Change posted by James Hartline on June 27, 1999 at 04:13:35:
Dear Rory and James,
At the moment I use Mobil 1 synthetic 10W40 in my 91 M5. I prefer 10W viscocity for numerous of reasons. First, it is not so thick, so the time the oil is in the cyl. head when starting cold is short without asking to much pressure from the oil-pump. Two, where I live, temperatures in Winter can drop to -10°C to -20°C. Believe me when you have this low temperatures you definitely want to use thin oil, 5W40 is better then. Thin oil protects your engine (especially the head) best on cold starts. If it is -10°C, you have a oil-temperature cycle of 110°C-120°C. In summer when +25°C, temperature cycle is 75°-85°C. So in winter you need the extra protection of thin-oil.
The disadvantage of thin oil (<10W40) is noticeble when you live in hot (semi-tropical) area's, there thick oil protects the engine better since 5W40 gets to thin on operating temperatures. I would prefer synthetic 15W40 then.
Last year in October-December there was a discussion going on the other M5 messageboard. For more details I refer to that discussion.
My conclusion is, when it is cold (In winter), use 5W40 synthetic, otherwise use 10W40. Only if you live in tropical area's use thicker oil. I generally switch oil to 5W40 in october and back to 10W40 in march/april.