At the farm we have a 55 gallon steel drum of SAE-30wt oil that is probably about 5-7 years old. My father who isn't an oil nut by any stretch of the imagination runs it in our (gasp) Ford 6700 diesel tractor, '83 Chevy 1 ton w/366 tall block, and our '58 Ford Jubilee 4cyl tractor(gas). Is the oil still good after sitting in the drum for so long? Of the 3 above mentioned engines, which ones would benefit greatly from a different oil? I'm thinking the 6700 diesel and the 1 ton dually. Thoughts?