recently spoke to a certified oil examiner and was told high mileage oil could possibly the worst thing you could put in a good running engine. along with it swelling seals to help with oil leaks.
high mileage oil is just a mix of several different weight and viscosity oils most common are
0w30,5w30,10w50,10w30,straight 50 weight and straight 30 weight oil

Only way to confirm this, is to take 10 identical engines and run blind tests on them using multi types of oil. Otherwise, your brand X oil still tastes just like my brand Z oil.

Ya true. I got my information from an engine oil engineer that tests engine oils so I will never use the high mileage. if I'm worried I just put in a bottle of Lucas oil stabiliZer

And just for arguments sake, what scientific proof do you have that Lucas is better as an additive than high mileage oil vs regular or synthetic?

Me, I just put regular high mileage oil in. That'* it.

I don't know what'* in Lucas but I have used if forever and never had any issues with it. same things might go for you using high mileage oil and it works forever. I just know that high mileage oil is made to swell seals in your engine. and I have no idea why that would ever be good for your engine over a long period of it'* life. all I know is if a oil specialist tells me not to use it I won't. and he doesn't work for an oil company either. it'* a no biased company

I am not so sure that it is just swelling the seals or more so treating them, not to dry out and become brittle. Fact- Over time, your gaskets & seals from pressure, heat,cold, and overall driving conditions will eventually dry out somehow. Additives like PAO, ester and god knows what exists in oils to treat these problems. Unless you can find a list of chemical compositions for each brand, it really is a shooting match.
When synthetic oils first came onto market, cars started leaking like a siv, cause they did not know the older seals would act in that manner. Syn oil manufactures had to put additives in the oil to stop older seals from drying out /shrinking and then...In new cars the composition changed in the seals, allowing for more long term flexibility with use of the syn oils.
Regardless of what oil I may use or blend myself, GF-5 rating is a good starting point.
Main GF-5 Site Testing
And when it comes to high mileage oil, many do not meet GF-5, mobil1 super 2000,high mileage does, and Pennzoil...check your bottles...before buying.

Oops..Here is the performance section link I should have posted first on GF-5
Main GF-5 Site Performance
That site has a wealth of info, if u are willing to read and spend some time.

the engine doesnt need any lucas anything, all you need is a decent quality oil. i run my cars a long time and i dont use any add on crap. i tried high mileage oil one time and both cars ran like they had winter gas in them and got 2 less mpg. all i do is regular Valvoline every 5k. and they always get the same or better mileage as new at 2 to 300k.

i usually dont run lucas but there was a closeout sale on it a few years back so i figure ill use it up! lol currently i run 1 year 25,000 mile signature series amsoil in all of my cars.
used to use valvoline synpower"full synthetic" but found out its actually not full synthetic or atleast thats what i was told

i have found some info not on persay high mileage oil but many tests done on many different oils done by an independent testing agency. check it out

http://www.amsoil.com/lit/g3115.pdf