Motor oil is not pure. They all have additives or additive package blended with it before bottling. This is most certainly true for any Multi-vis oil sold.
Whether oil is synthetic or "natural", the only real difference is the uniform size of the molecules, synthetic's are more uniform.
The additives or detergents are probably of the most concern in high mileage engines. If oil used previously was low detergent and the new stuff high detergent, then chunks of old oil/sludge/debris can drop off and clog the oil screen, leading to oil starvation. Then again, that might NOT happen.
The other additives of concern might be those that help soften rubber seal. A change to that *can* have effect on oil leaks.
One possible scenario is the detergent cleans away sludge to expose a leak/weep. But, continued use softens the seal and it goes tight again.
There are endless combinations. And I'm pretty sure the additive package formula Fram has added isn't published. Further, Fram unlikely makes the oil, it is just rebranded from some other source. Fram may or may not even control what the additive package is in the oil they sell. It's up to the distributor agreement contract, which you probably can't ever see or gain knowledge of.
IMO, Fram is far more interested in selling/promoting the brand than actually having a quality product. But, accidents CAN happen!
I do use their filters on my SOHC4s. But, I do my own QA inspection on them before installation. I don't have that luxury with most other Fram products.
Cheers,
