You can mix up CHEAP 10W-30 oil from Walmart as premix, and it'll probably work well for years & years, too. Doesn't mean it's the best way of doing things. I guarantee you'll be having carbon build up enough to choke the saw down to where it won't start, let alone cut anything. Same with Stihl oil - carbon buildup keeps their service department fat and happy. Like I said, they're not in the business of having your saw last forever.
A good synthetic will have next to no carbon build up. My neighbor buys Saber from me and runs that in his Stihl - he cut 10 acres with this ONE saw, and it's NEVER been apart. I've never had to do any carburetor tuning either. The built-in fuel stabilizer in Saber works great. My customers that insist on running OE or cheap dollar store oil are here all the time getting carbon scraped and burnt out of mufflers, spark screens, piston crowns, and exhaust ports after so many hours. If you cut 2 or 3 branches a year, then 10w-30 is the oil to use - it'll work. But if your WORK your saw, there's no replacement for top shelf synthetic oil.
To each his own.