Paul, it isn't quite as easy as the mechanic suggested. The amount of oil needed depends on the design of the engine and the quality of the oil. There are three main reasons to try to minimise the amount of oil: reduced air pollution from the burned oil; less carboning-up of the exhaust port, muffler, and piston ring grooves; and extended life of the spark plug. Generally, the major parts of crankcase induction 2 strokes probably will last longer at 25:1. The 50:1 figure is mainly based on the least oil you can use and still get a respectable life from the major parts of the engine - but you will also get less exhaust port and muffler fouling, less tendency to get ring-sticking, longer spark plug life, and if the carburetor has been calibrated for 50:1, slightly more power.
As you can see, this is not simply a matter of "the mechanic is always right" or of "the manufacturer is always right". Leaf blowers usually spend quite a lot of time running at idle or low power, and these are the worst conditions for getting carbon build-up in the engine. If you were a professional forester and this was a chainsaw that you were going to run hard and often, the position would in my opinion be tilted a bit more toward using some extra oil, but for a leaf blower in home use, I end up with the standard recommendation we just about always give at Outdoorking: unless circumstances are exceptional, it's best to follow the manufacturer's recommendation. In this case, that is 50:1.
