Tom--

If all else fails one should follow the instructions. So, maybe I should do what they say.

But, the reason I have been using Mobil 1 is because it is supposed to be more stable vs temperature & time and should have superior oxidation resistance, maybe superior lubrication. I don't know if any of this is true.

The reason I am thinking of going to Mobil 1, 0-40 is because it should have a more stable viscosity versus temperature than the 15-50 I was using and will be slightly lighter. This will make it easier to shift at low temperatures while maintaining enough viscosity for adequate lubrication at high temperatures.

I checked my manual for my '84 P'up and it calls for 10-30 or 30 or 40 weight oil depending on the ambient temperature.

Regards--

Paul