KW,
The best extended drain Mobil 1 product is going to be their 10w-30, Supersyn. The 0w-40 isn't nearly as shear stable and uses the same basic additive chemistry. It won't offer you any better protection or performance. The M1, 10w-30 uses little or no VI modifier, so the engine will also stay cleaner, even with long drain intervals.
If this engine is in decent shape, I'd also recommend running the highly friction modified, Amsoil Series 2000 instead of their 10w-40. Again, the 0w-30 is going to be more shear stable (it uses a even better VI modifier), and it's about 10% thinner in terms of HT/HS viscosity.
Your peak oil temps will run 5F-10F lower with the 0w-30 - a very good thing in Florida - and fuel efficiency will be about 3% better then w/ the 10w-40. The Jeep 4.0L engine responds very well to the type of borate FM's that Amsoil uses. I think it will be more cost effective to use than the 10w-40 - even with a drain interval of only 12k miles:
12,000 miles @ 20 mpg = 600 gallons of fuel
600 gallons @ $1.50/gallon = $900.00 of fuel annually
3% fuel savings with the Amsoil 0w-30 = $27.00 saved annually.
Price difference in seven qts of the 0w-30, vs seven qts of the 10w-40 is approx $17.50 annually. ( Assumes a six qt initial fill and one qt makeup oil)
Note: I'd actually recommend buying the Amsoil 0w-30 in a case of 4, one gallon containers (TSO-04). This saves you fifty cents per qt over the same oil sold in a case of qt containers. When you see how well it works in this application, you'll want to run it again ....trust me on this one.
Tooslick
www.lubedealer.com/Dixie_Synthetics
[ November 27, 2003, 06:07 AM: Message edited by: TooSlick ]