What's the best synthetic oil to use?

I tow 8k to 11k lb trailers about 6,000 miles per year, local and cross-country. What's the best stuff to use?

[ 08-21-2002: Message edited by: ksevern ]</p>

There are choices here and you will no doubt get about 322 responses in the next 4 hours on this one...
I am admittedly quite biased (experiential bias, however) toward the Mobil Delvac 1 5W-40 full synthetic. I work with a number of fleets that run the oil 100,000 miles between changes. Several fleets have every single truck with over 1,000,000 miles and no bearing change-outs and most importantly, no turbo-charger losses.
Delvac 1 has been around for over 20 years with a proven track record. Flows to 60 below zero yet provides extraordinary protection in 100 degree heat. Good availability,
Highly recommend doing an oil analysis prior to switching to it for a baseline for use with determining best drain interval.
George Morrison

ksevern
The 2 big dogs in the synthetic oil industry are Mobile 1 Delvac and Amsoil...You will not go wrong with either of these choices...
As with george morrison I am biased but toward the latter...do some research however stick with 100% synthetic after the breakin period [img]smile.gif[/img]

As mentioned above, it's a toss up between Amsoil and Mobil.

While many have had good luck with the 5w30 Amsoil, I'm a little skeptical. My next analysis with 40w should tell me more.

I must concur with the choices of Mobil Delvac 1 and Amsoil. I use the 15w-40 Amsoil year round and like it alot. I've heard nothing but good things about 5w-30. But I wouldn't use it unless you are in a constantly colder climate.
The Delvac 1 may be your best bet though. 'Cause you can get it anywhere. Well, almost anywhere. Moreso than the Amsoil. Not to mention that it is a great oil.