+1 Ppl think a HiMi is some exotic formula but in fact it's more similar to the plain formula than it is different. As you pointed out it's more "robust"..more or better seal conditioning agents, usually higher ppm total of AW additives and dispersants/detergents. Some HiMi formulas even have higher than avg TBNs, presumably to combat the blow by that older engines experience. In short there is nothing in a HiMi formula that is going to grenade your engine or existing seals or as I've suggested before, oil mfgs a) wouldn't be recommending it for even new engines and b) would have to slap a disclaimer on the bottle saying "Do Not Use Unless Your Seals Are Leaking"...... and fwiw, asking a seal conditioner to stop an already blown out seal is a tall order which it may or may not be able to do depending on the size of the leak..they (sca's) work best prophylactically - preventing the seal from becoming dried out and cracking to begin with. One might ask, "how does a seal become dried out in an engine"?.. indeed a good question, over time varnish and sludge can coat a seal preventing it from being exposed to the lubricants conditioners..in time that seal can shrink and crack..so the detergents in a HiMi are designed to clean out that sludge and varnish so the leaking seal can once again be exposed to the SCA's.. this process can take some time; several thousand miles.
Originally Posted by SubieRubyRoo
MolaKule has gone on the record stating that high mileage oils generally have a more robust antioxidant package than a non-hi-mi version, along with other "good" stuff (usually extra detergents and seal conditioners). He even said it may be preferable to run high-mileage oil from day one to maximize its benefits; and considering that 99% of the time, the high-mileage version is the same cost as the "regular" version, what downside is there?
Originally Posted by billt460
I'm beginning to think "High Mileage" oil is somewhat of a snake oil. I'll only add that I would love to be proven wrong about this.