I have 5 of these engines (old type OS MAX) and they show varying degrees of "coating" on the piston, from a "new old stock" one I am currently running in, to very well used ones (including one which is now in the "spares only" bin as it is clapped out and has a lot of big end play). Last Thursday I flew both all three models that I have that use the OS40MAX and the OS25MAX engines on 5% nitro / 20% castor, with added Castor (Southern model craft brand) to make it up to 25%. I used the same plug (OS 8 ). I found that the run and the nice 4-2-4 break was still fine, no issues arose. More muck on the wing however (as expected). I did not detect any variation in temperature on landing, but was only using the finger on the head method!!
In view of this I have decided to play it safe and keep it simple, by just adding castor to bring it up to the OS recommendation of 23% to 25%, I will use 25% as I am not after every last bit of power and wish the motors I have to last.
I did the maths and adding the extra 5% castor brings the Nitro content down to 4.8%, which I very much doubt makes any difference from the 5% original.