bought the murray new 6yrs ago. 20hp/46 cut Hydro..been running great,but now seems to like keep slowing down the longer you mow..will travel fast first 15-30 mins then just seems to go ony at a walking pace (slow).. I know not much on hydros..not making any strange noise so to say.. Have the manual on it BUT nothing for the hydro..all I can seem to see is a rubber tube with screw in it.. Is this the oil filler?? and how much and what type??Its not leaking any oil and the belt is good/not wore..Belt not slipping as far as I can tell..Is this worn out,low on oil,filter?? I mow maybe 1 time a week,,perfect level yard, and maybe 1hr when I mow,1 acre. Nothing else wrong with this at all.. Any info would be helpfull.. Thanks,Ross