Gengine, I just had a thought with a JD I worked on last week. This may not apply to the 3020, but it does apply to any JD which has a fuel tank mounted low enough that it uses a lift pump to get fuel through the fuel filter and over to the injector pump.. Come to think of it, this applies to the injector pump as well, not just to the lift pump.
Both those pumps are driven off a cam and the only thing that keeps oil and fuel from mixing is the seal between the cam and cam follower in either pump. So that seal does much the same job as a valve stem seal. The difference is that if the pump follower seal goes bad I'd expect the crankcase to probably get fuel into the motor oil.... which you would notice on the dip stick.
I think you said the motor oil was lowish, so that probably isn't the problem. But since it didn't run very long it still could be. It is worth thinking about if the valve seals don't pan out or if you see a gain in motor oil level in the sump.
As you say, all 4 stroke engines are basically the same.
rScotty