Popgadget
Elite Member
I need to replace the transmission oil in my Gravely garden tractor.
Over the years spec changed from straight 30 to 10W40 and now 10W30 motor oil.
am wondering if better to use a semi synthetic which might be less likely to leak at old seals.
Rotella 5 or 6?
old straight cut gears, don't get hot, probably some slop in them.
I would suggest 15/40, no need for synthetic, but use a high quality oil for the better additive package. The transmission in the Gravely doesn’t get hot enough to boil off moisture, especially if it’s not run all day, so an oil with a good additive package like the higher API ratings would be helpful. I think the current top rating is SN.
Keep in mind that a 10/30 and a 10/40 are the same viscosity cold, just that the 10/40 has a higher film strength. That’s why I suggested the 15/40 so it would be a little thicker.