Hi,
strangely enough here in Germany Stihl has replaced the full synthetic HT Bio plus (or was HT plus Bio) that I used up until now with a semi synthetic called HP Super. I don't know why though. Very strange. Anyway the HP Super now has a fuel stabiliser added which wasn't the case with the older full synthetic or the standard red mineral oil (I only use this for running my engines in anyway these days).
I always found that the standard red mineral oil left a lot of deposits, even at 2% mix, but the synthetic left everything really clean even though I use it in a 3% mix. Now that the full synthetic is gone I will either have to give the semi synthetic a try or look for a suitable full synthetic from another manufacturer such as Husqvarna or maybe Motul.
I find the different marketing quite confusing at times. Some dealers here say the red Stihl oil is synthetic, which it isn't, it is only a Standard API-TC mineral oil. The old full synthetic didn't say what standards it reached, if any, and the new semi synthetic has ISO-L-EGD which is very good. Husqvarna of course has had ISO-L-EGD oil on sale for years.
Still all of this probably only applies to Germany but I should imagine that the oils are the same in America and that the marketing people have always been playing around to make the old oils appear better than they actually were. Maybe someone could tell me if the new "fully synthetic" over there is actually just the semi synthetic HP Super?
Bye
PS here in Germany Stihl ONLY recommends 1:50 (2%) for Stihl oils or 1:25 (4%) for others oils of "dubious" quality, although I did get Stihl to admit to me that e.g. Castrol RS2 or the full synthetic Ester based oil from Motul are just as good or better than the "standard" Stihl oil. Wasn't easy going to get an answer though...