What magical qualities do you think semi synthetic possesses that would cause this? Synthetics do not “cause” leaks. They may, however, clean off the accumulated junk on a seal that was already damaged that non-synthetics do not… are you saying that you prefer to leave accumulated deposits from subpar oils in your engine to “seal” leaks, but where they can also cause other problems?
It likely has to do with the inconsistent molecule size of the semi synthetic base oil that "clots" the seals. I never said synthetics "cause" leaks, that is definitely a myth. However, they can let oil leak past worn seals that conventional oil doesnt leak through as readily. This is a phenomenon that others on this forum have discussed. The difference in leaks on my vehicle between synthetic and semi symthetic is night and day. I have tried to switch to synthetic 3 times now and each time I have majorly regretted it.
Also, there are very few accumulated deposits in my engine. The nemco 0w40 is diesel grade and has a very significant detergent package. I have had the oil pan off and there was no sludge, just the tiniest film of oily residue, despite being 30 years/ 270,000km . It is extremely clean under the valve covers, almost spotless. I know the entire oil change history for this vehicle, it ran on mobil1 5w30 from 1993 to 2021 and was always changed on time. So acumulated deposits is not the issue with this engine. The rear main seal stops leaking on conventional or semi symthetic, it isn't "magical", it's just a side effect of the lower quality base oil. I know that my single example doesn't prove anything however I can assure you that switching over to synthetic oil directly correlates with my rear main seal leaking pretty badly, and after switching over to semi synthetic there is only a small drop every couple days.
Last edited: