Originally Posted By: Jim Allen
The engine in the 3120 is a Yanmar, IIRC, and they aren't picky. The CI-4 will be fine. If you are using ULSD (very few people aren't these days, especially if you buy it from the diesel pump at the local fuel station and not special farm fuel), you can safely go with the CJ-4 (which is easier to find). Your tractor may be a few years old, before the ULSD was mandated, hence the CI-4 recommendation. I'm using this combo (ULSD/CJ-4) in several tractors that are much, much older than yours and it's been working out great (verified via UOA).
Shell or Delo??? Tossup. I'd be the indecisive guy at the two-holer outhouse if I had easy access and good prices on both. Here in Ohio, I find Shell more common and better priced.
Viscosity? FIrst it depends on how you use the tractor and the climate. Flagstaff is warm but not Phoenix hot and does get cold weather. If the 3120 just putters around a few hours at a time, personally, I would choose a 10W30 in your conditions because it will offer you good flow on winter starts, ditto on everyday starts during short hops when oil temp isn't up, but still plenty of protection. If you work it hard for hours on end, day after day, then maybe you have to step up to the 5W40 T6. IMO, the 15W40 is unsuited to winter starts or short hop operation. I would relent on that point if, A) the tractor was inside and B) was plugged in on cold days, but I think you would be better off with the 10W30 (the budget choice in any case, about the same cast as 15W40, though not as common) or the 5W40 (the more expensive one).
FWIW, I operate my utility Ford tractor (32hp) on Rotella 10W30, and it gets along just fine. I had winter starting troubles with it on 15W40 (didn't have a block heater then, now it does!)
Anyway, notes from the trenches from a motoroil-head with similar equipment.
Thanks for all the info.
I think I may run 15W40 during the summer and see how the tractor starts in the winter and then switch to 10W30 or 5W40 depending on how the engine handles the cold. I need to check and see if the engine has a block heater or not. It can get fairly cold up here during the winter nights, sometimes sub-zero. I want to use the tractor next winter for plowing snow among other things.
