Tractor Supply sells the proper “Fjord Tractor” sae 90 for these gearboxes.
Someone more qualified can speak to this, but I am beginning to suspect that some level below the plug is preferable. My Sm420 likes to find its own level out of the input shaft area. I don’t totally understand, but the early SM420s had some sort of non-sealing seal at the front….by design.
