False. CAFE is a consideration with every vehicle oil recommendation.
Not really, only ones that are typically mass produced. There are myriad examples, such as BMW spec'ing 10W-60 for M-cars, FCA/Stellantis spec'ing 0W-40 for the SRT's and HD 6.4's, Ford spec'ing 5W-50 for the Ford GT and the Mustang GT Track Pack...etc. Plenty of examples where CAFE considerations are tossed to the wind because the volume isn't high enough to matter.
The 5w30 viscosity is unchanged from the late 80s.
While the SAE 5W-30 grade as defined by J300 hasn't changed, the approvals, and subsequently, how the oil is blended certainly has, which is the point that is being made. A 5W-30 from the 80's was not subject to the same degree of testing as it is now. This is why we are now down to 0W-8, 0W-12 and 0W-16 instead of 5W-30 in the pursuit of fuel economy.
That said, any xW-30 post SL is constrained in formulation by the API limits on phosphorous, something that isn't the case with the xW-40's, which is why M1 FS 0W-40, 5W-40, Castrol 0W-40...etc all have higher levels of phosphorous despite now being SP.
The people who designed the Corvettes then didn't use that viscosity in their own Corvettes in the 80s and 90s. Now those Corvettes are 30+ years old. You can take a stab at convincing them to start using 5w30 now because formulations have improved
Why would the conversation be with them, when it is you that is here, having this discussion?
IMHO - a much more logical speculation would be "Since the Corvette Design Team and Lingenfelter Performance Engineering used 10w40 oils in the 80s/90s Corvettes, those engines may be protected even better by the premium full-synthetic 10w40 oils of today".
10W-40 in the 80's and 90's was likely a 30 grade 30 seconds after turning the key. The VII polymers used were crap, the oils sheared like crazy and they broke down quickly. They were also blended with cheap base oils.
They used what was available.
Things have evolved, and improved, considerably since then. The more recent Corvette Race program was using M1 FS 0W-40, just like many of the 24hr cars at LeMans and Sebring.
You have no data to support recommending 5w30 over 10w40 for 80s/90s Corvettes in any driving conditions. Because ... wait for it ...
...
...
... that data does not exist.
Let's look at some 10W-40's. Here's PYB 10W-40, note the 12.8% Noack. M1 0W-40 is 8.8% in comparison:
Note the API-levels of phosphorous, no higher than your plain-Jane 5W-30.
Kendall 10W-40, same deal:
Here's Kendall GT-1 5W-30 in comparison, doesn't look much different, Noack is better than the PYB:
Now, let's look at a Euro 5W-40:
Higher phosphorous and much lower Noack than all of them.
Oh, but let's really toss a spanner into things here and include a Euro 5W-30:
Look at that Noack! 6.2% for BMW OE 5W-30, and look at all that phosphorous!
Now, which oils do you think are going to be more shear stable and provide better service?
