I think there is little debate that manufactures are under the gun from EPA and burning lubricating oil as a side effect of 2 stroke combustion produces higher pollution numbers.
The established 50:1 recommended by manufactures minimizes pollution and keeps the EPA somewhat happy. But even to achieve this manufactures have found the need to spec their own oil and are not content to say any oil will work at 50:1, as they clearly state that if other oil is used it must be mixed at 40:1. Sure it likely sells oil overpriced OEM too.
But does mixing at 50:1 give enough protection, The answer is likely yes based on the number of people using 50:1 (roughly 45% based on the poll here) given that there is no error made in mixing measurements, fuel condition is up to snuff, OEM oil is used and the saw is in good order.
Does mixing at say 40:1 provide any additional protection?
Assuming saws run at a air fuel ratio of about 12:1 like most 2 strokes under reasonable load, There would be one weight unit of fuel and oil per 12 unit weights of air, of this one unit weight of fuel and oil 1 part would be oil to 50 parts gas by volume as it is measured. That works out to an air to oil ratio of roughly 663:1. Having I unit weight of oil per every 663 unit weights of air in good conditions has been demonstrated to work and manufactures stand behind this with a warranty.
Now lets say there is an air leak and 5% of the air coming in is bypassing the carb, there is a primary effect where by the air to oil ratio will go up to 696:1 which then effectively makes the fuel oil ratio equivalent to 53.5:1 instead of 50:1, but there is a second compounding effect, as the velocity in the carb drops so does the quantity fuel pulled in and the air fuel mixture right from the carb (the other 95%) drops and it will end up closer to maybe 12.5, this further hurts the air to oil ratio likely now more in the range of 720:1 or an effective equivalent to mixing fuel and oil close to 60:1.
This goes to say that a 5% air leak would reduce the lubrication at the cylinder walls and bearings by about 10%. Overall the air Fuel ratio has jumped from 12:1 to over 13:1, this increases combustion temperatures significantly as well as upping the RPM. The increase in temperature then reflects more heat down into the engine base and reduces charge density which then again reduces the amount of oil available for lubrication. Likely this heading towards air to oil ratio of over 750:1, equivalent to mixing fuel and oil at 65:1.
Two other bad things happen, as temperature goes up, the piston expands and clearances are diminished and to top that off the little lubricating oil left becomes less efficient as the viscosity and lubricity drop.
Even going with 40:1 fuel to oil would likely not keep the designed quantity of oil present with a 5% air leak, but it might be enough to keep sufficient oil with a smaller air leak of a percentage or two.
