It's based off of old experience, old repeated 'knowledge' etc.. it isn't applicable with modern engines and modern oils..
... the only potential cause with modern engines/oils is if the seals have already failed and are just being plugged up by dirt/sludge.. and fresh synthetic oil ends up cleaning said gunk out resulting in leaks.
Whaat? Good grief you people are gaga about the word synthetic. Mono-grade lubricant users are compared to global warming deniers and the Flat Earth Society.
Now if you believe in global warming, then why is engine oil going the other way?
Back in the day, 10W30 was the standard multi grade. Truckers wanted thicker and invented the wildcat grade 15W40, likely by mixing 10W30 with SAE 50.
Suddenly 10W30 isn't good enough, because in some climates 5W & 0W cold start protection is need in the winter months. Note; 10W is close to where 5W was 20 years ago.
Not good enough to have seasonal oil changes anymore, cold climates get 5W & 0W year round.
Not good enough that cold climates get 5W & 0W year round, everybody gets it, hot or cold.
This plays right into the need for synthetics and why there are no 15W20 or 10W16 oils on the shelf.
I think the best explanation of the reason "some people" like EricTheCarGuy might say it was in his conventional vs synthetic youtubes, specifically that part that deals with leaks. He took a lot of heat for his initial yt on conventional vs synthetic oil, so much so he did a follow up. I would say it most applies to much older engines that have run conventional almost exclusively. Synthetic oil doing more cleaning may tend to expose some areas not kept as clean by conventional, and thus leaks become more prominent.
Pretty good video. I switch back and forth between mono-grade, multi grade and multi grade synthetic depending on expected ambient.
The engines outlast the rest of the vehicle.
My difference of opinion from Eric as to why switching to synthetic from conventional, is that one base oil type cleans up after the former.
For example putting SAE 40 in an engine that has run years on 5w30 conventional, or an engine that has run on synthetic,
but somewhat neglected, may run hot and have a burning smell for a thousand miles or so.
The oil may turn dark quickly. Oil leaks might occur, but I have never witnessed that.
Unlike the boutique oil sales people, Eric is pragmatic and not trying to sell you on promises.
I watched Eric's whole video and after posting, saw how wordy and long winded he was/is and decided against posting. I was mostly referencing the leak/(noise) portion. For those interested though, here's the link to his follow up youtube. The 1:50 min. mark talks about leaks. Eric's other observations and conclusions conventional vs synthetic regarding viscosity and flow, I take with a grain of salt.
Way back it was not uncommon to have some crud keep seals from leaking on older engines. The owner would add synthetic oil which would clean away the crud and behold the seals would start leaking. In any case, almost all modern vehicles today use synthetic or syn-blend.