A few issues here..
I have spent lots of time on this issue. My partner has been building race engines for 16 years. We have personally done lots of experimenting, testing and researching oils as well talk to the top tech guys at the various companies.
First we need oils that can go one weekend for our dirt track guys that run big and small blocks and are limited to using only flat tappet. The Valvoline is easy to get and cheap but the best part is that it has proved to be the best in this scenario.
Second we need oils for the vintage guys. Cobra's, Corvettes, Jaguars etc that also have flat tappet. After much conversation and susequent testing, we found that most feel that changing their
oil often is far less troublesome and costly than pulling their motor and paying for a rebuild.
The Valvoline Race
oil will begin to break down after 500 miles due to the lack of stabilizers so as a precaution, Valvoline suggests that limit. But again, that's for racing. On the street I am sure it is fine for longer but again, cheap insurance.
When you are at a bar and you order a Scotch and Soda expecting it to taste a certain way, there is only so much room in the glass for the components Scotch, Soda and Ice. A quart of
oil is the same. You can't have everything. A friend used to have a sign. Good, Fast, Cheap. Pick 2. You can't have it all.
The race oils lack detergents and stabilizers but the benefit of the extra additives far outweigh the length they will last given the use in a scrub, flat tappet.
Also, if you use a strong detergent modern oil in an older rebuilt flat tappet now you have other problems. All those years of deposits in the pan and all over the motor will kick loose. The low detergent race oil will not disturb the sludge. Much better.
Also you mentioned sitting in the car being stored.
If you had 100 miles on the race oil and stored the car for 6 months, you could still go another 400 miles. The oil only breaks down when you use it. But, for the heck of it, I take every car that has been sitting, take it out to get everything warm, drain it and replace all fluids anyway.
Back to cheap insurance.