Which rear axle oil

[Blue)(Fusion]

I read the latest recommended service manual from Ford and see that 75W-140 is the recommended gear oil for severe usage (sustained WOT above 45MPH or towing) and 80W-90 is the normal use recommendation and factory fill on non Police Interceptor model Crown Vics.

I'm flushing and filling two cars so want to know what to get. One is a '99 Crown Vic with 3.27s open with 65k on the clock and the other is my '00 Marquis with 108k on the clock but replacing all bearings and seals and installing 3.27 gears with a Trac-Loc.

Both are daily drivers, but the Marquis does the occasional quarter mile at the track.

Would 75W-90 (synthetic form of 80W-90) be satisfactory or should I fill them with 75W-140 (synthetic). The 75W-90 is thinner so fuel efficiency should be a little better than with 75W-140.

[fleettech]

75w140 and limited slip additive.

[lights_sirens]

I'm running Mobil 1 LS 75W-90 and it's very quiet back there. Great gas mileage with a 3.27 locker in my '98 P71 and not all that expensive.