My Disco is my DD, off-roading happens, but not nearly as much as I'd like. I was thinking I'd use a full syn 75w90 (Mobil 1's winning the battle for my wallet right now) for the-hopefully-better MPG. (I also might soon be taking a job requiring a 60 mile daily commute, I have a real desire to squeeze every foot per gallon I can!) I'm not going to harm anything using the OE weight am I? Thanks for the other tips!