Best Gear Oil for your Deuce and a Half Transmission and Transfer case

Published on July 24, 2021
Duration: 7:38

This guide details the correct fluid selection and maintenance for the transmission and transfer case of a Deuce and a Half (M35) military truck. It emphasizes the critical need for non-detergent, straight-weight motor oil (SAE 40 recommended) or GL1 90 weight gear oil that is 'yellow metal safe' to protect internal bronze components. Using modern detergent or multi-weight oils can cause significant damage due to the lack of an oil filter in these older transmissions.

Quick Summary

The best oil for a Deuce and a Half transmission and transfer case is SAE 40 straight weight, non-detergent motor oil. This is crucial because these older units lack oil filters, and non-detergent oil allows contaminants to settle. Modern GL5 gear oils should be avoided as they can damage bronze bushings; look for 'yellow metal safe' or MT1 designation if using gear oil.

Chapters

  1. 00:00Introduction: Deuce and a Half Fluids
  2. 00:07Engine Oil Considerations (Rotella, Zinc)
  3. 00:52Finding the Right Deal: SAE 40 Non-Detergent Oil
  4. 01:06Why Non-Detergent Oil is Crucial (No Filter)
  5. 01:34Deuce and a Half Transmission Challenges (Bronze Bushings)
  6. 01:58Yellow Metal Safe & MT1 Designation
  7. 02:36Original Spec: Straight Weight Motor Oil vs. GL1 Gear Oil
  8. 03:14Gear Oil vs. Motor Oil Weight Equivalency
  9. 03:42Napa GL1 90 Weight Gear Oil Alternative
  10. 03:56Shifting Performance with GL1 Oil
  11. 04:15Fluid Capacity and Quantity Needed
  12. 04:43Conclusion and Call to Action

Frequently Asked Questions

What is the best type of oil for a Deuce and a Half transmission and transfer case?

The recommended fluid for a Deuce and a Half transmission and transfer case is SAE 40 straight weight, non-detergent motor oil. Alternatively, GL1 90 weight gear oil that is specifically 'yellow metal safe' can be used, though it may result in harder shifting.

Why is non-detergent oil necessary for a Deuce and a Half transmission?

Deuce and a Half transmissions and transfer cases lack oil filters. Non-detergent oil allows contaminants to settle at the bottom, preventing them from circulating and causing wear, unlike detergent oils which hold them in suspension.

Can I use modern GL5 gear oil in my Deuce and a Half?

No, modern GL5 gear oils are not recommended for Deuce and a Half transmissions as they can attack the internal bronze bushings. Even some GL4 fluids may cause issues over time. Always look for 'yellow metal safe' or MT1 designation.

What is the difference in shifting performance between SAE 40 motor oil and GL1 gear oil in a Deuce and a Half?

SAE 40 straight weight motor oil generally provides much easier and smoother shifting in a Deuce and a Half transmission compared to GL1 90 weight gear oil. GL1 oil can make shifting, especially into second gear, significantly more difficult.

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