my 88 560 SL with 100k miles does the same thing. if the injectors leak out the fuel in the system, when hot the increased pressure in the system drives the fuel out of the system, requiring a bit more cranking to get the injector prossure back up. I usually give it about 1/4 throttle, and it starts right up.

this is a bad situation to let go. i confirmed it when i changed the plugs out and they all smell heavily of fresh gasoline when pulled out 20 minutes after shutdown. all that extra gas is either soaking ontop of the intake valves and boiling off leaving varnish and gummy crud, or it is ending up on top of the piston doing the same, or worse leaking down the side and washing oil from the cylinder walls and drying or gummin up the rings. this should be addresses of you can confirm the leaking by a smell / look test or watching the fuel pressure fall with a gauge after shutdown.