What are the specs on the Evolution oil you are using?
The first batches of evolution oil were actually Mobil 1 which is good stuff. More recent batches of evolution oil are totally different. Its blue in color and i suspect is morgans coolpower.
Nearly all 2-stroke oils are designed for just that, 2-strokes. They are not designed to lubricate valve trains and they tend to evaporate or varnish up on the exhaust valves because gas burns hotter than glow.
Try the Stihl HP Ultra, for 2 reasons.
Stihl make 4-stroke pre-mix engines similar to our model engines and the ultra is the oil that they specify in those engines.
It has the highest flashpoint of any 2-stroke oil i have found. Its not going to evaporate or varnish as quickly as other oils.
There are hundreds of anecdotal reports from happy Stihl ultra users getting good results.
Lastly 20:1 is not necessary in the OS. It had needle bearings on the conrod so 32:1 is fine. Saitos need 20:1 due to their bushed conrods.