Ford 9 inch rear axle oil.

[Daytona Dek]

Originally Posted by likorplf

If you're experiencing noise from your 9-inch rear axle, it could be caused by worn bearings or resonance from the rigid suspension. Using EP90 gear oil is standard for most 9-inch axles and should provide adequate lubrication for the bearings. Thicker oils like EP70/140 or EP140 may help reduce noise by damping vibrations, but they can increase drag and heat, potentially affecting bearing performance. If the noise persists after switching oils, consider checking or replacing the bearings, as worn bearings may contribute to the noise.

Thanks for that. I didn’t know either of those things. That thicker oils can affect bearing performance or that thicker oil can dampen vibrations from stiff suspension.
As an aside I’ve got a shush shush noise on the over run in certain gears on my roadster which I’ve put down to harmonic resonance through the anti-role bar. One of the cures for that is to fill the role bar with expanding foam apparently but I haven’t got round to that to see if it works. I have got a roll bar in the Daytona which could also be bringing resonance into the cockpit.