I have to admit that most of our vehicles leak more than they
burn, and with seven vehicles to tend to I'm happy if I get
one oil change on each per year. They certainly don't get
changed by mileage (some, like the mostly-off-road Land
Cruiser, barely covers 1000 miles in a year!).
The ones that don't leak get synthetic - Mobil 1. The ones that leak
a lot get Castrol, usually 20W-50, since they leak because
they're old and tired.
I'm a big big fan of synthetics in the tranny and rear end. I use
Redline MTL or MT-90 in pretty much every gearbox, and various Redline
diff lubes depending on application. The Subie is the exception -
so far, it has what the dealer wants to make the warranty happy,
but I'm about to switch it to Amsoil "Smurf Oil" - Series 2000 -
because it's a gas mileage king, I don't shift it hard, and
we put about 1800 miles/month on that car...
See http://www.cobbtuning.com/tech/gearoil/ for a brief but
intriguing discussion of gear oil tradeoffs.
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