The two factors that either add components to or change the character of
oil are oxygen and water.  Oils have additives to prevent or moderate
the oxygenation and acid forming processes that result.  Synthetics may
be somewhat immune to these processes or contain a larger quantity of
the additives to give them their longer drain intervals.  If time wasn't
so short you could send in a sample from your crankcase and have it
analyzed.  This would be excellent info, and I, for one, would use
synthetics if the time interval were increased commensurate with the
mileage drain interval.

-Dana-

Bill Davidson wrote:
>
> Okay, other than around the block warm ups, my van has been sitting for two
> years... oil was changed to Mobil 1 (100% synthetic) two years ago. I'm
> about to head off on a big trip. The miles don't justify changing the oil...
> but I seem to remember something about changing the oil after a certain
> period of time regardless of miles. Should I change the oil before heading
> off on my big trip???
>
> Thanks,
> Bill