IMHO-you can buy a gallon of basic gear lube for the price of a small bottle of synthetic. The risk of getting water in the diffs is high and I would rather change it frequently than hesitate to change because I spent so much on synthetic and thinking it should last because it is better. Any fluids with water and dirt are not good. The milky look will tell you if it had water but I change it twice a year so it does not even get challenged. Must say though that I am not racing nor a performance machine and if I was I would run synthetic. And as some folks will not spare any cost for their 'baby' then the best is the synthetic.