Hyster H50XM transmission slippage

It's a 1994 model. Forklift just came back from the forklift mechanic but he is a Nissan guy, not use to working on Hysters. After new transmission oil and filter it now slips when trying to drive up a ramp when machine is cold but no longer slips when warmed up. Did not do that before fluid change. Could he have used the wrong oil to cause this. Some Hyster models use J20C spec Deere oil. Does anyone know if my H50XM uses the same oil?

Hyster H50XM transmission slippage