Pram - here's some news for you. Every piston-engined aeroplane, especially air cooled ones, on its first flight of the day takes off with a near-cold engine. It takes a long time at high power for the entire engine, and its oil, to get up to even temperature throughout. You can't achieve that in ground running, especially with an air-cooled engine. Some bits will get very hot and will be under-cooled on the ground, while, other bits will stay relatively cold, so it's bad practice to ground run a cold engine for long periods.
This is why an aeroplane that is grounded for a long time should never be dragged out, engine run, then put away. If you start it up, fly it! If it can't be flown, don't start it up!
Get it into the air once the oil is hot enough, and get the whole engine warmed through without the very hot spots (heads, exhaust valves etc) getting too hot.