Chainsaw Repair

I bought 15X30X7 single lip seals for my 801. I think they were National seals. The OEM seals were single lip. There are double sealing lip seals but they are made in a few select sizes. Almost all of the double lip seals have only one sealing lip and that is the inner one. The outer lip is a dust seal. They don't seal any better than a single lip seal. I haven't heard of anyone who had an issue using single lip seals without the dust seal lip on a chainsaw.
Viton is supposed to last longer than nitrile but is stiffer and is not near as forgiving if you have any radial run out.
Most of the nitrile seals that originally came with the saws have lasted many years. Some over 30 years old so I stick with the plain nitrile replacement.
If I can get new aftermarket seals I will buy them instead on NOS. You don't know how the old seals were stored and they could have most of the life gone out on them through deterioration. I am hesitant as you seem to be to buy seals if \I don't know who made them. There is lots of questionable stuff coming out of China.