Do you mean the engine begins to die when worked hard?
Fiddling the choke should not affect oil pressure, other than by bringing the revs back up. If engine is dying under load, could be that fuel supply is insufficient. Try cleaning carb. jets, check level in float chamber, fuel filters(usually one on tank pickup also), and that the tank is clean as rust and sludge can accumulate in the bottom blocking the filter.
Check that the governer is free and working correctly.
Loss of oil pressure when hot may be due to worn pressure relief valve, weak relief valve spring, or worn oil pump. It can also result from general bearing wear.
Hope this is of some help!
Mr. Toad, July 2008
