It appears that 5W20 will work fine on new cars with tight tolerances ... as long as you don't extend the interval too long.
As we have seen, many 5W30 SJ/SL "dino" oils will shear down to a 5W20 after just 3,000 miles. So, many people who bought a 5W30 are essentially using a 5W20 whether they intended to or not. This sort of goes along with what
TooSlick said above. It won't shear down much ... but will oxidize and thicken with extended use. And of course, wear will go up considerably at this point.
By the time 5,000 miles rolls around, these oils have pretty much had it. Sorry, but I can't say that double-digit wear metals are good results.
So, if you have a car that calls for these oils, I'd use them ... but keep the interval short ... even if the stuff says "semi-synthetic" and has moly. I would start at 4,000 and test every additional 500 miles until I got a result I didn't like.
Oh, and someone told me that Red Line
does make a 5W20 ... but you have to ask for it. I have no idea what the minimum quantity might be, though.
They make some ultra-thin race oils for qualifying, though. Stuff like 0W2.
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Bror Jace