Wood'nCoal wrote:JC says to use 75-140 synthetic oil.
That would work just fine. Out of all the gear oils on the shelf, 75w140 will have the highest viscosity index. i.e. It resists thickening in cold temperatures and won't thin as much at high temperatures as a 75w90 or 80w90 from the same manufacturer. I think using a synthetic gear oil is a good idea, but I admit that I'm running conventional 80w90 because I have a 5 gallon pail of it in the shop.
More important than the exact viscosity is keeping everything full of clean oil. My stoker is on track to run 1000+ hours this season, and I will include an oil change in the spring service.