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That's interesting. I've also wondered why 10W-30's are still on the market when a 5W-30 will have the same viscosity at operating temps and then even be thinner at cold start up. Seems like 10W-30 is obsolete.
My Mazda owners manual allows 5w30 and 10w30 across most temperatures that i'll ever see. I've experiemented with both and have not found a great deal of difference. The oil on the dip-stick "seems" thicker with 10w30 when the engine is hot, but ive had no problem starting my car in 10*F weather with 10w30 and no problems using 5w30 in 105*F heat. I track my gas mileage and also notate what oil is in the car at the time along with what octane is used. I've found no measurable difference that can be duplicated.
I'm somewhat hesitant to use 5w30 all the time in my car as previous (pre 1997) Fords and Mazdas with my engine have limited the use of 5w30 to temps below 65*F to 86*F depending on which owners manual you look at. Then again newer SM oils are a LOT better than oils 5-10 years ago and while the engine may be the same the oil pump design may very well have changed to allow for "thinner" ols.
The main benefit to using 10w30 in my car is it's a lot easier to see where the oil level is on the dip stick
For no apparent reason the oil my Mazda seems to like the best is plain old pennzoil dino 10w30. Friday it was 12*F outside, my windshield was covered with frost, and the sprinklers in my apt complex had coated the hood with a nice sheet of ice. My Mazda cranked right up like it was 80 degrees outside (If you dont count the power steering pump whinning for a few seconds).
The only Turbo i've had experience with was a 1985 Volvo Turbo that had over 250,000 miles on it when sold. It had pennzoil 10w30 dino it's entire life, changed religiously every 3,000 miles, and ran perfect when sold.
So 10w30 may be obsolete, but it's still not a bad choice for many cars that do not see arctic temps on a regular basis.
