Greg asks for real life experience; I've got some.
First off - some Corvettes (ZR-1?) *do* come with Mobil 1 in the
crankcase from new. I suspect that these engines are broken in on a
dyno or motor stand, since they come from the factory "never started".
I've switched to Mobil 1 on two cars in our fleet: the 1981 FIAT Turbo
Spyder, and my 1970 GT6+. We've been through several turbos on the
FIAT, and wanted to get away from anything that could possibly coke;
Mobil 1 seems the best compromise between price and quality (if price
were no object, I'd use Redline, since it has a higher flash point).
The GT6+ is my autocrosser, so it sees severe duty. After 4000 miles
with the Mobil 1, it has started leaking at the front seal. This is not
uncommon with synthetics in older engines - they clean away the gunk
that has been deposited in the seals and then find their way through
the smaller cracks. I'll be replacing the seal soon.
I have not put Mobil 1 in the TR4A, since her oil pan leaks away a
quart a month. Her engine gets rebuilt this winter; after breaking in
30 wt, I'll put in Mobil 1.
I will put Mobil 1 into the 1275 in the Morris as soon as I get it back
together.
I won't be putting Mobil 1 into the Ferrari, because the folks at Mobil told me
not to. Why? Because it will likely start leaking, as the GT6+ did. That engine
has not been apart in 27 years. I know that the front seal is new, because I
replaced it a month ago, but I'll bet the rear seal is original. I'm not
interested in a leak there. If/when we have to do a clutch, I'll try to
do the rear seal and then maybe change.
I'm not crazy about the idea of extended change intervals, just because I see
how black the oil gets from blowby at the normal 3000 mile interval. Yes, it's
expensive, but so are rebuilds. I view new oil as very cheap preventative
maintenance. If your oil isn't black at 3000 miles, then I'd consider changing
just the filter. If it is...
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