oldsaw
"Been There, Milled That"
Synthetic 20/25 -1 ,never been a prob ,no smoke......and no headaches.......Although not particularly nice when the breeze is throwing any exhaust AND chips in your face, with 20/25 or 40 :1.
you'll notice in your owners manual recommended
25:1 other oils
50:1 Stihl
rec for NORMAL USE - MILLING NOT NORMAL USE
As you may know, milling is hard on the saw.Your running it flat out for maybe a 15 minute cut....depending on the log size and wood type !
Other guys may use 40:1 milling...their choice.I use 35:1 normal use.
I milled with a 66 mag for a few years.....mostly HW.....40:1 Stihl.......
was my pride and joy...regularly serviced...blew it up......was rebuilt .I don't specifically blame it on the oil .
When i check my jug or plug , after a good milling session, on 20/25 they're not carboned up ,as some may imply.
Paramount is the tuning.....MS660 lovely saw...you'll love it.
All the best with your new milling venture.
Most of my milling has been oak, cherry, and walnut, with some hedge, hackberry, cottonwood, and other stuff thrown in for good measure.
The 25:1 "if you don't use our oil" is basically to make the saw so smokey and smelly that you buy their oil. It also makes their oil seem more like a bargain. There isn't reality there, only marketing.
Mark