I was reading the May '06 issue of Road & Track magazine today when I came upon a Technical Correspondence letter that related directly to my question about 5W-30 vs 10W-30 oil. What are the odds of that happening?! It's even about a Subaru. Maybe I should buy a lotto ticket! Here it is:
Oil Weight Quandary
I've been using Redline oil in my 2002 Subaru Impreza WRX since its original break-in period. The air temperature where I live rarely dips below freezing. I understand 5W-30 is recommended but chose 10W-30 due to its tighter viscosity range, which should mean fewer additives and less chance of lubricant breakdown under extreme conditions. The car has 60,000 trouble-free miles including eight or ten track days. Would the lighter weight oil be better for the engine?
I suspect this is probably overkill, but both oils are offered, so there must be a difference.
We'd stick with the slightly heavier weight 10W-30 for the reasons you cited. Plus, if you are track driving, the narrower viscosity range should perform better with the elevated oil temperature you're sure to get at the track, and cold-start lubrication should not suffer because you aren't subject to truly cold starts. The very thin 0W and 5W oils are relatively modern lubes aimed mainly at improving fuel economy reducing internal engine drag.
So I guess I have nothing to worry about and will probably continue to use 10W-30. My only decision will be when I make the switch to synthetic.
Another article "Your Mileage May Differ" was also a very good:
For enthusiasts, the gasoline that we put in our cars' fuel tanks is liquid entertainment. We enjoy driving and like to do it enthusiastically. But there's no reason we need to be profligate with fuel. That is, there are plenty of things we can do to enhance fuel economy without diminishing the enthusiast value of our motoring.
I've read lots of articles about saving gas, but none were applicable to the way that I drive so this one was particularly interesting and feels do-able.