I am going to help my son work on his M818 tractor - it seems that fuel is getting into the oil - the dip stick reads over a gallon high. What could cause this problem? What should I check? The truck starts and runs OK at this time. Oil pressure starts out good but as the truck heats up the pressure drops quite a bit, I think because of the thinness of the oil. Any help will be appreciated, thanks in advance.