I am a new member with a 05 Base XLR that sat for a long time. I am in the process of replacing all the fluids, hoses, belts, water pump, etc. I am starting on the trans and diff, I have changed the oil on a 05 STS sealed trans and know how to do it. The XLR trans called for Dextron III on 05. I was curious if I could use a synthetic(like mobil 1) without causing any damage, and if it would be better than Dex III. Any insight on this would be helpful.