I have used fuel containing 10% total oil and 10% nitro on my OS FS-52 without problems. The plug was OS F type.

Its the Modeltechnics Dynaglo fuel with 8% synth and 2% castor. Isn't it?

To be honest I have run this engine on this fuel with up to 25% nitro again without problems. Perfect idle at 1,800 rpm, transition and peak power.

Prop was MA 12x6.

For sure the difference is in the superior quality of OS engines compared to clones like SC.

Stelios