Originally Posted by Dave9
If you are not losing oil due to seal shinkage or wear, a HM oil with a seal conditioner that swells them more than their already acceptable amount, will only wear them faster and should be avoided.
It's not "any reason not to" so much as don't use HM oil unless you have a reason to. An engine with no problems should continue using the same high quality oil you would have used. Mileage cannot predict the HM oil need, some engines last many more miles than others plus there is the driving environment, whether they were easy highway miles or short trips and/or temperature extremes.
HM oil does nothing useful for a properly working engine (and exhaust). If it did, they'd put the same additives in all their oil.
Rubber seals can and do get brittle, crack and shrink over the course of tens of thousands of miles. The idea is to PREVENT a leak, not try and fix one after it's occurred. At that point you're asking a lot of a HiMi formula to undo years of neglect and the window of opportunity to stay or undo the effects of time, has essentially closed on you. In that case you can't blame the HiMi formula for not being able to undo the fallout of your poor choices. There was a time when you could do something about it, now it's too late....