How much of a problem is it if you over fill with oil and how much overfill can you get away with? The manual just says it might cause a problem - it doesn't say it will cause a problem.

 

A little over the top hash mark will not cause any problem. If it's way past that then you may experience oil getting to your air filter and MAF sensor causing idle problems.

 

Where the dipstick turns round again is roughly 1/4 quart over filled and you won't have a problem with 1/4 quart too much oil, most dealerships us the rest of the quart that fills it up when doing an oil change anyways, it will eventually burn it anyway.

 

Try running through water thats 1 foot deep, then try running through water thats up to your neck.

Gives you an idea of why overfilling is detrimental to the performance of the vehicle.

I don't know what happens to a rotary exactly, but in piston based engines, excessive overfilling (over a qt extra) can result in the crack whipping the oil, creating foaming, and foam don't lubricate very well, nor can it get pumped by the oil pump. Lots of bad things happen then.

 

Yeah, donno about the rotary, but overfilling the oil in a piston engine is a worse idea than underfilling it. The cars I have owned have all warned of serious engine damage in the manual, which may be a hint that the rx8 isn't terribly affected.