Lyle - I posted this somewhere else, a while back. It is my own experience but it convinced me of something I've always suspected - that 29% oil is just a waste.
I'm convinced that 22% will do no harm (I run it in virtually every engine I own) but there is an advantage to 29% - read on:
“For anyone curious about lubrication of a Fox .35, here’s an experiment that I did a few years back.
I had two identical Fox .35s, the first version with the fore-and-aft muffler lugs on the case. One was brand new in the box, the other had a lot of time on it. The head was totally encrusted with burned castor to where the screws were nearly covered. The piston was black. Compression was strong and there was virtually no slop in the front bushing. Although I bought it as such, I assumed that it had lived on a high castor diet. The only thing I did to it was to remove the baked on castor using Dave Gierke’s “Demon Clean”. A fine product.
I broke in the new engine in on Powermaster 5% nitro, 22%, 50/50 oil for approximately one hour. I then put both engines on a variety of different aircraft, switching them around from time to time. Both profile and inverted mounting. I tried a tongue muffler as well as a Fox muffler with the exhaust nipple cut off to enlarge the hole.
I ran both engines on the same Powermaster but with 10% nitro. I used the engines for two seasons accumulating hundreds of flights on each.
The piston of the used engine lightened up a bit but the compression NEVER changed – it always had a good “snap”. The new engine ran perfectly the entire time. Neither engine showed any adverse wear in the piston/cylinder fit or the front bushing. The piston of the new engine still looked new.
My only complaint on both was that they did not exhibit the classic Fox 2-4 break. On a whim, I ran some 5% nitro, 29% straight castor in the newer engine, which was then mounted in a Nobler. Instant classic 2-4 break – exactly as I remember.
Switching back to Powermaster, the break went away.
Anyone else have a similar experience? By the way, I still have both engines and they have excellent compression. I would not hesitate to put either in a classic or old-time plane.
Bob Z.”