Hello, Tried to search this but no luck. I have a 1992 E30 BMW Convertable It has 130,000 miles on it. It has always Lived in Tucson AZ, and has always has Castrol 20W-50 oil. The inside looks remarkable, but the oil and filter has been changed every 2000 miles. I have a BMW Manual that says 20W-50 Summer 10W-40 Winter. All of the actual BMW Mechanics in Tucson said to run 20W-50 Summer 10W-40 Winter. I have the car in Indiana now. Not so hot outside. My Mechanic here said less than 20W-50 Summer 10W-40 Winter will cause the high mileage motor to rattle, and wear. He said if I run anything lighter than 10W-40 it must be Synthetic, and that it should only be for short times. It is even in his shop manual. My Brother is completely freaked out. He thinks All Beamers should run 5W-30 synthetic. I would like your opinion, and why you think this way. When I change my oil, it is still really clean, and the inside under the Valve Cover is Super Clean. Engine doesn't use any oil.Runs smooth. 2.5l Engine.