A reader asks: I have a 2008 Dodge Ram 2500 with the G56 six-speed manual transmission. My truck is fairly stock. I’m looking to replace the ATF fluid in the transmission with gear oil. Mercedes builds these transmissions and recommends MobilTrans SHC DC SAE 50 transmission oil in Europe. This product is hard to find in North America. Do you have an oil that matches up?

G56 6-Speed Manual Transmission

The G56 manual transmission for Ram Heavy-Duty trucks. (photo credit: dieselhub.com)

Our answer: Chrysler’s suggestion of ATF+4 automatic transmission fluid in G56 manual transmissions has been an ongoing controversy since this manual transmission was first added to heavy-duty Ram trucks back in 2005. But despite the internet chatter, the experts we spoke with all agree that ATF is still the best option for G56 transmissions. AMSOIL Technical Services and G56 experts like Midwest Transmission Center Inc. agree that there isn’t any upside to using thicker transmission oil. For over a decade, Chrysler has steadfastly maintained that an automatic transmission fluid meeting their ATF+4 specification is correct for the G56. Consider also that Ford has been suggesting ATF in the manual transmissions of their Super Duty pick-ups for several years. This is not a situation unique to Ram G56 transmissions.

Why Is ATF A Controversial Choice For G56 Transmissions?

The G56 transmission is made by Mercedes-Benz in Europe. North American Ram owners have noted that a heavier oil is suggested for European trucks equipped the G56. This is why many Ram users explore the option of using a 75W-90 or even an SAE 50 transmission oil. Some internet sources are actually making the suggestion to overfill the transmission with heavier oil to dampen noise and enhance protection. Heavier oil and over-filling is a recipe for elevated transmission temperatures.

Overall, a common assumption is that Chrysler chose to use ATF in the G56 because it was cheap and readily available. The other commonly held belief is that ATF is simply too thin to provide adequate protection. We see no evidence that this is the case. Remember that Mercedes and Chrysler were part of the same corporation when the G56 was first mated with Ram trucks. After exhaustive testing, Chrysler could have specified any oil they wanted, but they still suggest ATF+4 to this very day. If ATF was causing havoc in G56 transmissions, there would have been a specification change by now.

Why Did Chrysler Go With ATF In G56 Transmission in North America?

Four reasons.
1. Capable wear control
2. Smooth shifting
3. Resistance to extreme cold
4. Cooler operation compared to thicker oils

While European G56 applications may use much thicker oil, North American trucks are exposed to extreme cold that isn’t a factor in Europe. Consider that both SAE 50 weight oil and 75W-90 gear/transmission oil have a drastically thicker viscosity than the ATF+4 recommended by Chrysler.

SAE 50 Transmission Oil

  • At 40°C, SAE 50 is 388% thicker than ATF+4
  • At 100°C, SAE 50 is 257% thicker than ATF+4

75W-90 Manual Transmission Oil

  • At 40°C, 75W-90 is 228% thicker than ATF+4
  • At 100°C, 75W-90 is 185% thicker than ATF+4

Remember that gear oils use a different viscosity scale from engine oils. A 75W-90 gear oil can fall into the SAE 40-50 weight range.

As the numbers above show, both SAE 50 and 75W-90 are a lot thicker in viscosity than an ATF+4 transmission fluid. In cold weather, ATF will provide much smoother shifting performance.

Why Some G56 Transmissions Fail

The alleged G56 transmission problems generally do not stem from oil related issues. A common problem is clutch failure due to increased horsepower (from major engine mods) or heavy towing beyond the load capacity of the truck. Transmission experts we spoke to insist that the use ATF is not a cause for transmission failure in G56 units.

Will Thicker Oil Dampen Vibration?

The G56 transmission coupled with the Dodge Ram heavy-duty chassis and drive train can be prone to noise and rattles. But this has more to do with the various drive train forces and mechanical activity resonating through the aluminum transmission body than the viscosity of the lubricant. Thicker transmission oil has a minimal effect on issue.

Does Oildepot Sell An Oil For Ram G56 Transmissions?

Our best option for the G56 manual transmission is AMSOIL Signature Series Synthetic Multi-Vehicle Automatic Transmission Fluid. This ATF+4 formulation ticks all the boxes for what is needed in a G56. AMSOIL ATF is formulated to resist shear-down and oxidation in the face of extreme heat. This keeps the transmission free of deposits and varnish. At the other end of the temperature spectrum, Synthetic Multi-Vehicle ATF has a cold pour point of -53°C. So shifting is easy and consistent during extreme winter weather. If you’re going to use ATF, why not use a world-class formula? We can sell AMSOIL ATF at wholesale in Canada and the USA. Contact us using the form on this page for more information.

We would be happy to sell you a thicker oil. But unless you are drag racing, sled pulling or have extreme modifications, our research suggests that ATF is just fine for normal operation.