Tractor Repair: Hydrostatic transmission slippage., transmission slippage, contamination risks

Question
I have a Honda 48" 22 hp tractor mower that pulls fine for about 30 minutes, but looses drive power after that.  It makes a whining sound under load similar to a worn or underserviced power steering pump.  The oil level is proper.  The owners manual specifies a multigrade engine oil in the trans. and reccommends against changing it due to contamination risks.  I'm wondering if I put a 30W oil in, I might get better performance.  It seems as if it's just worn and needs a rebuild.  Also, are rebuils an economical alternative to replacing the unit.  Everything else is in great shape.  Thanks

Answer
Jay by the sounds of it there could be a bearing going in the trans assembly before it dies and you need to spend mega dollars on it you should strip it down and find the problem.
This will save a lot of time and money and yes some times it is not a viable proposition to rebuild some times new is cheaper.



            Regards Danny Last