I take the approach that I don't know what is in there until I look. Someone mentioned that they change it if the vehicle is new to them. That is what I do so I know that the rear end most likely won't be a problem for me or the next person. I "assume" that the rear end grease has been in the rear end since it was new. So 50-60 year old used oil needs to be changed. I do it with out hesitation. I don't run synthetics in my old trucks either. I'm not saying synthetics are bad. Just saying I don't go through the expense of synthetics for my old trucks.
