GEAR OIL RECOMMENDATIONUpdated 2 months ago

Recommended Oil

• Lucas 85W140 non-synthetic (dino) oil is preferred.
• Use a Ford or Dana limited-slip additive if you have a clutch-style posi.
• For extremely cold climates, use Lucas 80W90.

Why Dino Oil Over Synthetic?

• Dino oil keeps metal parts cooler, even though synthetic may keep the oil itself cooler.
• Synthetic oils can thin out when hot, reducing their ability to cling to gears—this cling provides cooling and protection.
• Dino oil tends to maintain pressure better and cushions gear contact due to its viscosity and cling.

Water Separation

• Dino oils separate from water better than synthetic. This is important because water contamination reduces oil’s protective qualities.
• With synthetic, water and oil tend to mix—bad for diff longevity.

Cost vs Benefit

• Dino oil is cheaper and works well if changed regularly.
• Instead of spending more on synthetic, install a temperature gauge and monitor the diff’s heat.
• Change frequency matters more than oil brand—heat breaks oil down over time.

Temperature Guidelines & Oil Change Intervals

New differentials may run hot (250–275°F), but 300°F is too much. Let it cool before continuing to drive.

Typical Diff Temps (Based on Use)

• Stock application: 170–220°F
• Towing, big tires, smaller diffs: 200–250°F
• New diff break-in: Up to 275°F is normal

4 Most Important Oil Attributes

1. Cling ratio (how well it sticks to gears)
2. Water separation
3. Cushioning under pressure
4. Heat resistance/temperature control

Final Takeaways

• Frequent oil changes > expensive synthetic oil
• Use a temp gauge to monitor real performance
• Avoid synthetic unless solving a high-heat or shock-load issue
• Always change oil immediately if water gets in, and use proper venting to prevent moisture