There is no need for a blanket statement like: Synthetic will burn less than Dino or vise versa. One should compare the spec sheets and try to make sense of the outcome. For example, you should correlate less oil consumption with: Higher flashpoint, lower NOACK volatility, higher viscosity @ 100C, less aggressive driving/coasting, less high-speed freeway commute, less fuel dilution, and/or, a clean air filter.
Havoline 10w30 does have a higher flashpoint than M1, but not the Valvoline 5w30. So it could be anything. An educated guess says you need a more robust oil for your Subie Turbo. Any one of these should be less volatile than M1:
AMSOIL ATM 10w30
Valvoline Maxlife Synthetic 10w30
Valvoline Synpower 10w30
Havoline Synthetic 10w30
SuperTech Synthetic 10w30
Castrol GTX High-Mileage 10w30
Meijer High-Mileage 10w30
Havoline 10w30
That happens to be my stockpile right now, trying to lower my D16Y8 Civic motor's consumption from 1qt/2k miles. I already did 2x Auto-RX, and tried a 15w40 HDEO (Esso XD-3), no luck. I've got 125,000 rev-happy miles on the clock. Good luck.
EDIT: Out of the 125k miles, over 100k of them was on M1 5w (and later 10w30). Not anymore. M1 is just not cutting it for me. My intake manifold keeps getting really black and gummed up (crank-case ventilation of fuel and oil vapours) with M1. Upstream of the butterfly? Shiny silver Aluminium. Downstream? Yuky black soot. And I have NO EGR!
