Another synthetic I think worth investigating is Redline. They offer the usual weights 5w30, 10w30, etc. However, they also offer gear/bearing oil in various weights which is said to be safe with rubber. This oil is the MTL, MT90 and 75/90NS. They are sulfur free also. The MTL is about the same viscosity as 10w30, the MT90 about a 10w40. They have all the low friction properties of synthetics in fact I prefer the shifting in my car with the Redline after trying Mobil 1, Valvoline and others. I don't know if there is sulfur in motor oil. There is in gear oil. Jeff
