Well, I think you are getting the term engine seal and what types of oil causes leaks mixed up. An engine seal could only be compromised when you use synthetic too early and not conventional, in general. Do you have a high mileage vehicle or something because maybe it was referring to using Mobil 5000 on older cars -pre ILSAC GF4 perhaps even pre SM?? You know, old school technology? As far as a conventional oil affecting seals, it's like Mark twain -style, 'the two shall never meet' kinda thing. Are you sure they were talking about Mobil 5000 and not Mobil 1 which is synthetic and I've followed religiously the rule that never put full group III, IV, V in an engine newer then 20 thousand miles and I still haven't touched Full Synthetic (GRP III, ...) in my car and I'm at a little over 26k now?
All I can tell you is that Honda V6s take real well to liquid engineering. I don't know if it's the parts they use (more malleable surfaces etc or its some lightweight material) but I can tell you that's its geared and made well for 5w20 oil even though in the previous year to mine, I belive they were using 5w30.( I am referring to 2005 and older Accords)
If Mobil 5000 then perhaps it was in the context of a older technology and oil (GF 3, SL etc) and not the newer vehicles that spec SM and ILSAC GF4. ?
And if by engine seal you mean the ones that cause leaks. Because of anything else besides the engine seals is swelling, that's a good thing according to me because my car is performing all the better as I hope I have not reached the pinnacle with Motorcraft. What do you all think? Beating Supra engines, S200s and Eagle Talons and getting 25 mog?? I can tell you that as of last week, I hit 26000 miles but on my current OCI, I am at 4000+ miles on MC after using nothing but:
Mobil 5000 (broke in; first oil change)
Mobil 7500 (2 cycles; completed at 21,634 miles)
Motorcraft (current and 'ironically' best performer?)
ohh, what cracks me up is that even though its marketed as syn blend, its priced like a conventional. a big 'heh' in caps goes here.
And I am using nil oil on my current OCI and I am like at 4300 miles on current OCI so that's pretty good. The Mobil 5000 and 75ks only used about a hlaf qt per OCI so if that's what you mean then wouldn't I would be losing oil now, on my current cycle? But I am not.
I think we need more information as to if you were really meaning M1, mobils synthetic brother.
You see
Mobil 5000=conventional
M1=synthetic ('synthetics', in general; known arguably to cause leaks when used to early..
)
If I was a betting man, I would say what you heard was probably in reference to perhaps an older year vehicle who tried to use it or maybe even in terms of a used oil analysis report or reference of that maybe said something to that affect (if taken to the extreme-kinda used oil analysis thing)
hope that helps..