Obvioulsy the oil goes thin when hot, when you stop the engine it's draining away and takes much longer to pick up. The answer is got o a thicker oil or strip the engine and fit new crank bearings and/or oil pump.
I'd recomend the Lucas oils from E-Bay, they come with an oils stabiliser to prevent the problem you are getting and they do a good 20w/50. One £25 bottle has 9 1/2 litres so should last you all year.