Ok, so I've gone synthetic...

[Eric_R_Kissell at whirlpool.com]

With the miles I drive each car, I change my synthetic motor oil twice a
year - once in late Autumn when I install a Winter grade (10w30 or 10w40)
and once in late Spring when I install a Summer grade (10w40 or 20w50).

I like to change the oil filter every 5k miles using the specified Mann or
Mahle filter.

If I put enough yearly miles on each car, I would change oil every 10k
miles up to 15k miles with a good synthetic. I would go longer if I used
oil analysis to prove that I could do so.  You could probably safely go to
25k like Amsoil claims, but I would be more comfortable doing this if I had
oil analysis results that showed it was acceptable. Since I like to change
oil grades for Summer versus Winter, it really does not matter to me. I
have too many cars so the mileage I put on each is low enough I never need
to worry about going over 15k miles per oil change.

I use Amsoil synthetic motor oil. With my commercial account it is
reasonably priced.

HTH,
Eric Kissell
1991 200q 20v Avant, Koni Sports (yellow), 2.4 Bar (MRC), eBay Surprise
1989 200q 10v Avant, Boge Turbogas, 1.8 Bar (qlcc) - FOR SALE
1986 5000cstq Sedan, Bilsteins, 1.8 Bar (SJM)