Its no myth. It doesn't mean it'll work for everyone. The easiest method is religiously track the MPG and calculate ROI. Also, tracking common fuel system failures, within the vehicle family, that may possibly be avoided is a plus. Like any form of PM, this takes time.
I see it as, a simple cost effective way of improving the lubricity of the fuel. Very little goes a long way.
Your fuel pump and injectors should last longer with the extra lube. With the number of injectors and pumps that I've seen fail over the years, the only common denominator was that the owners never bothered with any fuel additives.
TCW3 oil not only lubes, but have cleaners in them to prevent carbon buildup in 2-stroke engines. Besides the benefit of improving the fuel lubrication, you'll end up with continual cleaning. If anything, you'll end up with less buildup on the pistons, in the CCs, and past the rings.
The improved cleaning caused by the 2-stroke formula should help maintain clean intake valves. As Shell says, stop gunky valve buildup.
Lubed valves/seats should last longer.
Any residual lube that makes it to the pistons/rings should reduce wear there too.
How good is your fuel blended? MMT, MTBE, ETBE, methanol or ethanol dosed?
You'll end up with more consistent performance over the life of the engine. Some people believe that degradation of performance over the life is acceptable. It, IMO, isn't. Learn to set some higher performance goals.
Besides the MPG improvement, I've quieted some noisy fuel pumps with a little oil added to fuel. That, IMO, was the only proof I needed that a few ounces per each fill definitely adds some amount of lubrication to the fuel system components.
Besides 2-cycle oil, Marvel, Lucas, FuelPower, Lubrigas, Biodiesel........can be added to improve your fuel. Remember that overdosing isn't needed. Start with an ounce per 4-7 gallons of fuel and experiment from there. Those 5 oz lucas UCL bottles are excellent for refilling doses. And, read the bottle directions.