I sometimes get asked how seal swell agents work and while doing some research on the topic, I came across this video from Lubrizol which shows how an oil's impact on seals is evaluated. The test shown here isn't the only one used, but it is one of the main ones.
Basically after being exposed to oil after a long period of time (a week at elevated temperature), the seal coupon is stretched to see if it has changed since new. High Mileage engine oils contain ingredients designed to soften older and harder elastomers and bring them back to similar levels of performance to new material using what is shown in these tests to evaluate the performance.
Loc: S California
I can't see changing to a HM oil regardless of mileage if there are no apparent problems. What's working should be left alone. There are those that must really abuse their vehicles for there to be a suggestion that 75K miles equates to high mileage. My 03 V8 4Runner is at 165K+ miles with no indication of excessive oil consumption or seals leaking. That's more than double this 75K cutoff and this engine works hard for its living.
Loc: Sauk Valley, IL
Very interesting. Iíve got some PPHM 0W-20 about to go in my Odyssey, and I have MaxLife 5W-30 in my Camry. Both are are high mileage vehicles and I like the idea of HM oils. Iíll stick with them going forward.
2001 Toyota Camry LE 4cyl. Valvoline MaxLife 5W-30 blend and FRAM PH4967
2010 Honda Odyssey LX Valvoline MaxLife 5W-20 blend and FRAM TG7317