fuel mix

I picked up a used, near new condition Stihl HT-131 (4-mix engine) pole saw for $200, yesterday. I was discussing fuel mix with the previous owner and he was telling me that he likes to use 20:1. Stihl specifies 50:1 and it goes against my grain to deviate from manufacturer's engineering...

Mike Acres site says 16:1. I assume that was with old SAE 40 oil. Generally I get some 40:1 pre-mix I run in other saws and haven't had any issue. Do I need to mix some special thicker stuff for this one since its a bit older vintage? This one is one of the first ones off the line in I think...