I have used some with no apparent problems, but unless you are buying an after market type specifically for a stihl part number, they are really hard to match exactly. Also, a few of them are proprietary sizes and nobody makes them for after market. I usually buy seals from Stihl (dealer) as part of the saws "set of gaskets" (cylinder, exhaust, carb if req; casing, seals etc). If you buy them individually, they run $8-10 each; Stihl will sell you the entire gasket set with Seals for $12-$15. Dealer don't tend to offer the "set of gaskets" as they make a lot more money selling each part individually, and my dealer didn't even know they existed until I gave them the part number!