Earl. The Amsoil isn't that much more than the M1 when you become a preferred buyer, even when considering the "membership fee." The Series 2000 is about $6.30/Qt, and their 10W-30 is only about $4.45/Qt. In my Jeep, I've extended the oil changes out to about 23,000 miles in a year (my average yearly mileage), to no ill effect. In my S70 T5, I'm starting at 5,000, and will do an analysis. I will probably extend to 10,000 miles or more with analysis. Before I extend, I want to get a grip on how the turbo chews up oil.

BTW, if your 850/S70 is turbocharged, I'd consider dropping the Series 2000 for the 10W-30. It's a bit more stable in turbo applications.
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1999 S70 T5