RE: OS Max 55 AX issues
12x6 at 11,700rpm is about where it should be. You'll never see the AX reach the advertised 16k rpms on large props. It's marketing hype.
Get some fuel with 18-20% of oil and at least 3-5% of that is castor.
"IF" the motor is truly overheating then the synthetic oil will be taken past it's flash or burn points and will burn away reducing the protection to your engine. Castor is the opposite, it protects more as the temp rises and will protect your motor from lean runs.
3 Turns out is a lot. It should be a definate 4-cycle burble at WOT on 3 turns out. Are you sure your not counting half turns as 1. I've made that mistake a couple of times.
If it's definately 3 turns out and running lean then you need to examine your fuel lines closely. Check the clunk line for holes. Make sure there are no holes in the lines between the tank and carb. Are you running a filter? They can sometimes leak or cause cavitation and air bubbles in the fuel lines.