I believe that while Mobil1 does not experience the same lubricating breakdown
and viscosity loss that conventional oil is subject to over time/temp, it does
serve the same detergent functions as conventional oils.  The oil's ability to
carry and/or neutralize harmful combustion byproducts is limited, so oil
change interval are based on more than simply dirty oil.  Of course I'm no oil
expert...that's just my understanding.  Seems to me 7500 is a good balance
between engine care and environmental waste.
FWIW, I used to run conventional oil until I did a minor rebuild on a Harley
at 35K and found grossly pitted cam lobes and grossly worn oil pump drive
gears.  After the rebuild I used only Mobil1 15W50 or Syntec 5W50(preferred)
and pulled the motor apart at 65K and found everything to be litterally like
brand new.  Can't say for sure if the oil was the only difference (or perhaps
my tremendous mechanical skill ;-) NOT!) but I now use nothing but synthetic
in everything I own...lawnmower excepted.
Mark

Austin wrote:

> At 03:59 PM 11/18/98 EST, you wrote:
>
> >Also I have
> >heard that if you change just your filter every 6k miles and just add the
> >quart of oil that comes out with the filter, that you can extend your oil
> >change durations from 6k miles to 10k miles or more...
> >Ken Wilford
>
> Somewhere I have one of the first magazine ads introducing Mobil1 (early
> '70s?) in which they were claiming oil changes of 25K miles/once a
> year...probably had to knock it off because of pressure from manufacturers
> (they now want you to follow mfgr's rec's)...hhmmmm, they sell more oil
> this way, right?
>
> Austin