It's like those drug commercials on TV...
"For some patients the benefits out-weigh the risks..." If the motor calls for 50:1 with the first tank or two at 40:1 that's not much difference; most stock motors are going to be slightly on the rich side anyway to provide some margin of safety.
I have nothing to back this up in any way, but it's my opinion that if your motor is designed to run 40:1 you'd be ok anywhere from 32:1 to 50:1. Changing the mix WILL change the A/F ratio so at least be aware of that. Now if your engine is designed to run 40:1 and you want to do something extreme like run 20:1 or 100:1 then be prepared to have fueling issues or possibly detonation until you re-jet to compensate for your changes.
EDIT: So I just went and read
an article by Gordon Jennings about mix ratios and it jived pretty close with the article I linked earlier. In his (1978) article testing mixtures of 15:1, 20:1, and 30:1. The article/test found 15:1 to be the best in terms of peak HP, engine cleanliness, and cylinder wear. The article also mentions having to change carburetor jetting for the non-OEM approved mixture ratios. In a nut-shell more oil=more lubrication BUT you
need to fatten up the carb. Less oil=less lubrication, needs leaner carb. No matter the mix there shouldn't be any plug fouling issues if your carb jetting is right and you have proper compression and spark.
Take it with a grain (or fist full) of salt. That article tested various mix ratios using the same Castrol 40R castor oil for all ratios. On the last page of the article they mention using non-castor oils at or very close to the mix ratio called for by the OIL producer, not the ENGINE manufacturer. A lot has happened in the last 32 years when it comes to oil chemistry and the materials and manufacturing processes used in modern engines, but I think the basics are still sound.
I just did a quick check and Amsoil has 2 different oils for different mix ratios; one oil for 50:1 and lower mix ratios, one oil for up to 100:1 mix ratio. All the Valvoline oils I saw are rated (by Valvoline) for use in 11:1 to 150:1 mix ratios. Klotz has one oil for 40:1 or 50:1, another oil for 32:1 or 40:1, another oil for 16:1 or 20:1, yet another oil that
specifically warns never to mix it oilier than 50:1
Ugh, Edit #2: So after re-reading the thread it appears (based on my research) that everyone that has posted this far is right (in different aspects).
I would say to the OP: Find out what your motor calls for. Look at the gas cap, or find an owners manual on the internet for
your brand of engine. Use the mix called out in the manual, OR add more oil to the mix, BUT adjust your carb accordingly if you want to run a heavier mix. If the engine calls for 50:1 mix then a 32:1 mix is likely to make it last longer (and maybe even make more power from reduced friction) IF you adjust the carb accordingly.
