Check engine manufacturer rating for continuous and intermittent.
For Safety, check the tractor manufacturers rating. So if engine says 40% but tractor manufacturer says 30%, than for safety purposes do not exceed 30%.
20% is only 11.46 degrees, so not much of a slope, I would assume most riding lawn mowers could handle that. Our units are rated at 30 degrees continuous (58%) and our Diesel engines are actually rated at 35 degrees continuous, but for safety purposes we rate them at 30 degrees.
So check with both engine and tractor manufacturer and I would take the lowest number. Safety should always come first! :thumbsup: