10% synthetic, 5% castor, 10% nitro.

Regarding heat at the exhaust, I've changed to an os flex and seriously, on the test bed, I can hold the exhaust just two minutes after stopping the motor. The ribs on the flex transfer heat away from the motor and exhaust like I cannot believe. And it solves all the exhaust vibration problems. Definitely worth putting on.
I'm thinking of adding a small flywheel to the propeller to help smooth out the ignition but can't help thinking that massive gear should already be doing that.

Keith, and I'm not being even slightly negative about the motor. Don't even think about running it in on the airframe. It's not even worth trying. During run in they are unbelievable messy, do your plane a favor, take it out anyway. This has nothing to do with issues. Just running it in. Mount it on a saw horse, take it out into a field somewhere, take a flask of tea, set of earmuffs and a couple of your favorite flying magazines.

Regarding gear backlash. I can't measure it but I'd guess, on a 15" propellor, the play is close to but not 1mm.

Cheers