Join Date: Sep 2006
Location: Las Vegas, Nevada
I ran Valvoline full synthetic with my original ford trac-lok limited slip differential without issue.
I was told NOT to run the full synthetic with the carbon clutches and stiffer s-spring in my rebuilt unit by my local builder and the ford fluid hotline. Currently I use 80w90 conventional with the ford friction modifier.
Contrary to what many think the modifier doesn't make the clutch type engage it actually makes it slip to avoid chattering and damage.
I was told that with the setup I'm using I would risk excessive slipping even with straight synthetic with no modifiers. Hence fully synthetic with a friction modifier would only be worse.
As far as synthetic being thinner theoretically it shouldn't be any thinner hence why the weight is the same, however it does typically flow faster than or looser then conventional. Which is one of the reasons why it is a better lubricant, on top of the fact that it resists thermal breakdown, varnishing, and foaming better.
-1996 Pearl White Thunderbird 4.6 Sport 4v with many mods:
Current Best: 13.583 @ 103.74 10/2014.
227hp/241tq measured on calibrated mustang dyno, tested at full vehicle simulation with inertia enabled, no number skewing.
-2001 White Ford F-250 Super Duty 6.8 V10, 4-door, 4x4, Limo Tint, BFG KO2's, Bilstein 7100 Remote Reservoir Race Bypass Shocks, 6" Spring lift, Kenwood Double Din, K&N Intake, FlowMaster Exhaust
Amber_Murder's Daily Driver.
-2005 White GMC Sierra 3500 6.6 Duramax, Dually, 4-door, 4x4, Limo Tint, BFG Commercial T/A's, Rancho RS9000XL Shocks, Kenwood Double Din, K&N Intake, Banks Monster Exhaust
Chris_Murder's Daily Driver.