Multi viscosity gear lube too thin?

[dlundblad]

I have an older Troy Bilt tiller that I bought used and am going through. I think it’s from the mid 80’s.

I found an owners manual online and it specifically says not to use any multi viscosity gear oils because they are too thin. SAE 140 was the factory fill, but SAE 90 can be used as well for a year round oil.

I wonder what the logic behind this was? Surely an SAE 140 was almost as rare then as it is today. I plan on using xxw140. Likely a conventional.

 

[xgmad]

SAE 140 was old school gear lube pretty sure somewhat cheaper than an sae 90, a xxw140 will do just fine in the tiller in the summertime, they are not too thin, even the conventional stuff takes way too much heat to shear all too much.
My pressure washer pump calls for SAE 90, I'm not going to go buy SAE 90 for this, so I used 20w50 gasoline motor oil, and the pump has stayed normal temperatures and has had no problems.

Gear oils have different weight scale than motor oil, so a 90 weight is an equivalence of a 50 weight, not thin at all.

 

[zrxkawboy]

My guess is that a multi-viscosity would not get hot enough in this application to reach its “hot” viscosity. IOW, an 80W140 would be an 80 (or close to it) in use due to operating temperature.

 

[xgmad]

80w equivalent is above 40w in an auto vehicle, and i just tried to figure out 140 and its like 100 weight oil