Try the 12x4.

The engine will rev higher, but the plane will seem to fly slowly to you.

Put in a flight or two and see if the engine deadsticks or produces the low compression you mentioned.

Thus far everything is pointing to an overheating engine.

The "black oil" is burnt synthetic or castor.

The loss of compression may be the result of too much expansion of the cylinder sleeve because of heat.

The engine should get up to about 280 degrees F, which will feel hot to you, but if the engine is properly cooled the prop shaft will only feel warm.

If the prop shaft feels hot, the engine is running far too hot.

If the 12x4 does not deadstick but you find the plane does not pull out of hovers, then try the MAS 13.5x4.

BTW: What is the all up weight of your plane fully fueled?