ORIGINAL: Jezmo
I have put many an hour on my converted Saito 91. The leanest oil ratio I could get away with was 22:1 and not because of the rod bushings. There is plenty of oil in the case to keep the crankpin and wristpin bushings lubed. The problem is the exhaust valve guide. There is no oil traveling up to the rocker boxes from the crankcase, all of the oil for the exhaust valve lube comes from the exhaust gases. If you cut the oil ratio too much there isn't enough and the exhaust valve will stick in the guide. Been there done that. The exhaust runs much much hotter on the gassers and it tends to dry the oil off the stem and guide pretty quickly. I made it a habit to pull the two screws and take the rocker cover off after each flying day and drop about 5 drops of high quality synthetic on the valve train and put the cover back on. Never had any more trouble after going back to 22:1 and lubing after flying. I was trying to run at 32:1 when the valve kept sticking. There are really no drawbacks to running the extra oil except the cost. Plugs (I was running the RimFires) lasted an easy 30 to 40 hrs and sometimes more. Good luck with yours.
There is a way to insure adequate exhaust valve lubrication.
Purchase a set of check valves W/inlet filter screen. P#SAI130T68A
Screw the inlet valve into the back plate, drill & tap the cam housing for the outlet, run a line from the outlet to the exhaust rocker cover that is D&T'ed for breather nipples (you will need 3 additional nipples besides the one from the backplate that was replaced by the inlet valve) P#SAI6519
Run an additional line to the inlet rocker box & then vent to the atmosphere. If you need the crankcase pressure to pressurise your fuel tank, run the pressure line through a bleed valve to allow some pressure to vent to atmosphere. P#SAI300TTDP102
This will assure that all excess oil from the crankcase reaches the cam, tappets, cam gear, then the exhaust valve stem and just for good measure, the intake exhaust stem.
If you run the engine inverted, you can probably eliminate the lines to the rocker boxes as plenty of oil will likely travel down the pushrod cover tubes to lube the valves.