How many miles do you now have on your engines and are you using a synthetic or a conventional oil?
High Mileage oils are USUALLY a heavy duty oil with EXTRA SEAL CONDITIONERS. They also benefit from a much better HTHS(usually 3.5 or better compared to a standard of 3.0-3.2) They will also usually have ZDDP levels of about 1000 compared to a standard of 700-800.
If you have been using a synthetic oil since day one, you are already getting seal conditioners, amount varying by brand, and as far as seal conditioning goes, the High Mileage oil probably would not benefit you very much. However, the better 100C cSt and the extra anti wear additives may well benefit you. The one with many leaks could possibly benefit from a 10W-40 High Mileage oil such as Mobil 1 High Mileage or Valvoline Maxlife synthetic High Mileage.
I have a 1986 Toyota truck that gets daily use. It has 300,000 miles and does not leak or seep any oil, anywhere. It does not use a drop of oil between OCI. From 0 to 20,000 miles it got Castrol GTX 20W-50. From 20,000 to 50,000 it got Castrol Synthetic Syntec 5W-50. From 50,000 to 300,000 it got Mobil 1 15W-50. So, for 280,000 miles of its life it was getting seal conditioners.
My wife has a 1999 Chevrolet Cavalier. It now has 202,000 miles. The day we brought it home, I switched it to Mobil 1 15W-50. It does not leak, seep, or use oil. This engine has been getting seal conditioners since day one.
If you do go to a High Mileage oil, I would highly recommend that you use either Mobil 1 High Mileage or Valvoline Maxlife full synthetic High Mileage. Change out the first fill at 3000 miles and change the filter at 1500 miles and at 3000 miles when you change the oil. These oils will clean your engine and you want to have the filter fully operational. After 3000 miles, do your thing on OCI. I believer shorter OCIs are better.
