Nedyob,
Shoot me an e-mail and I'll forward you a list of glow plugs and heat ranges that I've compiled over the years, mostly from contacting the manufacturers.

My experience with an OS 61 FS was similar to yours with a low oil all synthetic fuel too, it liked to run hot expecially with a 13x6 & 13x7 prop. I ended up liking very much a 12x7 or 12x7 on that engine. Try a fuel with 20% oil with at least some castor in it. 15% nitro sounds just fine with a medium heat range plug.

Fuelman