Yep, Sig. I get more flights using synthetic but it's still no where near 500. I have not tried Powermaster in years but maybe it's time to try again. I always felt like I needed to run a higher nitro Powermaster to get the same run as Sig. As of late I have been upping the nitro in Sig, the more I add the better it gets. My baseline is 15% now...
The 500 is just a WAG, I have had only one outright failure (apparently dinged when a bit of chipped bearing flew out, since it happened simultaneously with the engine suddenly sounding like a coffee grinder). Otherwise I will change the plug every few years just on principle, and I can't really point to any degeneration in performance.
Don't worry about the nitro now. For a while, the old pre-VP Powermaster was the same as SIG, but later they snuck down the nitro to maybe 7-8% for the "10%". VP fixed it, and it's back to full power now. Even if it wasn't, you just pick whatever you need and ignore what it says on the can. I routinely run the 15% VP Powermaster in the midwest. It was a little jumpy the last few years when it was cool, but really good when it was hot. I have occasionally gone down to 12.5%.
I generally take 3 gallons of 15, 2 gallons of 10%, and a can of YS 20/20 to I can mix anything from 10 to 20. I seem to recall using 17.5 for a few really hot days and have run YS20/20 for tests. YS 20/20 straight out of the can is one of the tricks for flying at Tucson and Ted ran it at the 2004 WC.
As long as you stay away from the 4-2 break, you can more-or-less run whatever nitro you want and not have to worry about compression. If you are attempting to run across the break, then, you might have to play off the compression and the nitro to get an acceptable mix of boost/brake and overall power. Every time I tried it, I found it a lot better to have lots of nitro and compression and no break, VS, less nitro and adjusting the compression for an acceptable boost/brake. It also needles very nicely with more nitro.
Brett