I was wondering what it would take to make an engine survive on 5w20 oil. It seems that Nascar engines all run really tight clearances(.001) and thin oils. Why couldn't a big block mopar do the same thing. I have to run 20w50 in my car due to .0025 bearing clearances. It sucks for cold starts and wastes horsepower.
It seems a lot of engine builders are "old school".....ie 1960's technology...and run loose clearances. What gives?

I use 5W 20wt Valvoline non synthetic in my Duster street motor (518 C.I.) and in my race motors also, have for ten yrs.+ now. I use standard race bearings on the rods and full groove Federal Mogul truck on the mains and end up with over .003 bearing clearances on both I also use a high volume oil pump and cut down the bypass spring so the motors have around 70 lbs of pressure with the oil at or above 160 F at or near 7000 RPMs, this combination will give me around 20 lbs at idle at or near 850 RPM in gear in the summer with the oil temps at or above 180 F The pressure jumps as soon as I touch the throttle up to or above 30 lbs at 1500 RPM BTW, I usually shift the street motor at 7000 RPM or higher when racing it at the track, it has a stock Mopar 1970-71 street Hemi 6 quart oil pan with a home made cranksahft scraper and stock Mopar windage tray, no bearing or oil control problems in four years on this combination

I`ve been running 5w30 Valvoline for a while now w/rod and main clearances in the .0024-.0034 range w/out any issues so far and this is a street car in extreme hot/cold desert conditions.