48F, 67, the 'best' oil is going to be an opinion, but I'll give you mine. Use mobil 1 15-50 synthetic. IF, that is, you don't have leak problems already. If you do have leaks, fix them before you put the synthetic in. Synthetic has never caused me any more leak problems than any other oil, but if you have a leak, synth is exspensive stuff to be dripping on the ground. While you're at it, how about Mobil 1 synth for the axles and trans and T-case aswell. Mobil makes a great 75-90 synthetic that can go in most gearboxes and axles. The advantage to the synths, as I see it, is how well they flow in the cold. I don't know about y'all, but winter here in Michigan can get pretty cold, one year at deer camp was -35* F or colder at night for a week straight. Mineral oil just doesn't like those temps. The synth doesn't seem to mind at all. DF, @ his Dad's house