I have to agree on the Sta-bil, I'm still using that same bottle and everything seems fine. That's one of the nice thing about Stihl oil though, it already has stabilizer so there's no need to mess with an aftermarket one. Their oil comes in three variant's; "Stihl HP" in the orange bottle which is a regular dino oil, "Stihl HP Super" in the black bottle which is a dino-synthetic mix, and "Stihl HP Ultra" in the white bottle which is a full synthetic. That said, when I bought my 600 last year the dealer I purchased from gave me 2 bottles of regular HP oil in the orange bottle. Already knowing that this oil is what specifically causes excess carbon build up in 4-mix engines, (carbon build up on the valves is the main culprit to a 4-mix engines reliability issues, which was the reason why Stihl developed their "Ultra" oil), I just stuck it on the shelf when I got home. Before I left the dealer they filled the tank with their mix, (I'm assuming they probably used regular HP oil), and after I ran that tank through I filled up with my regular use mix of Amsoil Saber at 80:1 w/ 89 gas. I have run that though the 600 for the past year, about 30 hrs worth of time, and it hasn't missed a beat and starts on the first pull in warm weather, second in the winter.
If it was me, I would buy the Stihl Ultra HP as soon as possible. It was design specifically for the 4-mix to curtail carbon build up, and it's only a few dollars as well so why not? On a side not, some people were confused, (including myself), about the statement on the Ultra oil bottle which says something to the effect of "Biodegrades by 80% in 21 days". Owners weren't sure if this meant "after 21 days it starts loosing is lubricating ability?". To fine out specifically what this meant, I emailed Stihl a few weeks ago and they stated that the biodegradability rating is based on "if it comes into direct contact with sunlight". For example, if you were to spill it on your sidewalk than it will eventually go disappear from the exposure to the sun, and not that it will loose it's lubricating abilities while mixed with gas in a storage can.