After finally getting the damaged plugs out I've put some more 75 / 90 into the diff. It has been making some noise and some vibration under acceleration, I think. It now seems improved some. So I'm looking to totally replace. Fuchs TITAN Gear Hyp 90 Hypoid Gear Oil GL5 80W-90 GL-5 80W90 4 Litres 4L 5031131237725 | eBay This looks like the right stuff. But I'm considering an additive to perhaps improve it further, assuming it has some wear at 123k miles. Muscle Car ANTI-FRICTION Engine Gearbox Diff OIL ADDITIVE Ametech METAL-TEC10 | eBay Any thoughts?