In the past the rule was: For partial fill-ups (a few ounces or less) use SAE 140, SAE 85W-140 or SAE 80W-90 weight gear oil with an API rating of GL-4 or GL-5. For full replacement use SAE 140 or SAE 85W-140 gear oil with an API rating of GL-4 only.
We have also found that 80W-90 oil with an API rating of GL-1 is acceptable for both partial and full replacement of tiller gear oils.
We have 80W-90 lubricant available (12 quart case) at our Online Parts Store as part number 737-04040.
This oil is also available at most automotive supply stores.
