Silkolene is a proper fully synthetic oil unlike Castrol. Not sure whether Mobile 1 is. Read Oilman's explanation threads as apparently some oils claim to be fully sythetic but are infact hydrocracked mineral oils. I've just switched from Castrol edge 10w60 to Silkolene Pro 10w50 and been very impressed. The oil pressure is the same and for some reason it didn't burn any oil at all on a 500 mile round trip where as the Castrol would have used a good 1/4 litre.