I have two Stihl 4-mix engines (FS110 and BR600). Both user manuals state to use Stihl 50:1 2-stroke oil OR a high quality equivalent. The BR600 manual specifically recommends (but does not require) HP Ultra fully synthetic, the FS110 manual is not specific. When I bought the BR600 the dealer included a 6-pack of Stihl Premium oil, which doubled the warranty. Stihl's US website mentions doubling the warranty period for certain equipment in non-commercial use if Stihl HP Ultra is purchased with the equipment.
Stihl Premium is rated TCI-AP on the bottle. Mine were made for Stihl by Shell Canada, no mention if it is synthetic, so probably not. No mention of fuel stabilization.
Stihl HP Super is a semi-synthetic rated JASO-FD, ISO-L-EGD and "Particularly recommended for STIHL 4-MIX engines" according to the Stihl website (I don't have a bottle to look at). Includes fuel stabilization. This product IS on the list of JASO-FD certified oils. Oddly, no mention of TCI-AP.
Stihl HP Ultra is a bio-degradeable fully synthetic also rated JASO-FB, ISO-L-EGB that includes a fuel stabilizer according to the website, but rated TCI-AP according to the specifications brochure and the label on the bottle. So maybe meets all 3? However, it does NOT appear on the list of JASO certified oils -- either as -FB or -FD.
Amsoil Saber Professional is a fully synthetic that states it is "for use in applications where JASO-FD, ISO-L-EGD or TCI-AP are specified" on both the bottle and the website -- but again, they do not appear on the list of JASO-FD certified oils. Their Technical Data Bulletin states that they themselves performed some testing. It does specifically state it is suitable for Stihl 4-mix engines. There is no mention of biodegradability. Given that Amsoil proudly touts this feature in some other products (eg: hydraulic oil), that probably means it isn't. They do state it stabilizes fuel.
Personally, I have used Amsoil for years in the FS110 and other 2-stroke equipment without problems or concerns. But I wouldn't want to get into a warranty battle over it since it is impossible to prove it is equivalent to Stihl's products. Amsoil's "Warranty Secure" icon means bumpkiss -- it's just a marketing gimmick they made up and has no legal standing. Doubly so outside the US, where the Magnusson-Moss act has no effect.
As for Stihl's products themselves, they are obviously all over the map. Stihl apparently offers a conventional, a semi-syn and a fully synthetic, any of which apparently meet their warranty requirements. Suppliers (and therefore possibly formulations) vary by market. The one oil that states it is specifically recommended for 4-mix engines -- and that IS actually JASO certified -- is not even mentioned in the manuals for either of mine! Stihl is equally guilty of playing an oil marketing game in claiming Ultra meets JASO-FD without actual certification to prove it. The only type of oil specifically forbidden in the manuals is TCW or NMMA grades that are intended for outboard motors (which are water-cooled).
But that's just my 2¢.