A frequently asked question is what oil to put in my NV4500 manual transmission? Searching the internet forums will yield a variety of answers that may or may not be correct for you. The goal of this post is to present information to allow you to make the right selection for your needs.
The NV4500 factory fill was Mopar Synthetic Manual Transmission Fluid SAE 75W85 GL4 Part# 4874459 for Dodge applications or AC Delco Synthetic Manual Transmission Fluid SAE 75W85 GL4 Part# 12346190 for GM applications. Both the Mopar and GM fluids are repackaged Castrol Syntorq 75W85 GL4. These oils have been discontinued but are still available through some dealers and are getting harder to find. This is the recommended replacement fluid if you can get it. What should I do if I can’t get this oil?
Before we discuss oil options, we need to cover some gear oil background. Oil used in an NV4500 must be GL4 or similar. Do not assume that a GL5 oil is acceptable to run because it meets GL4 requirements. GL4 and GL5 specifications are rated for differentials not transmissions and relate to the amount of EP additives in the oil. GL5 has roughly double the amount of EP additives like sulfur/phosphorous. The sulfur/phosphorous additive creates a coating on surfaces that acts as a sacrificial barrier the is peeled off while preventing wear on the parts. This is acceptable for steel parts where the coating is softer than the part. In a synchronized transmission the synchronizers are typically made of softer materials like brass or in the NV4500 a composite material. When the sulfur/phosphorous coating adheres to the softer material it is stronger than the base material. When the barrier is peeled off it also removes a very small portion of the base material. This will quickly wear out the synchronizers in the transmission if the wrong oil is used.
Oil that is too thin can cause shifting issues as the synchronizer will engage too quickly resulting in grinding or hard shifts. If the viscosity is too high the transmission may not shift until the oil warms up.
Option 1: ATF+4
Dodge dealers have now been instructed that the use of ATF+4 is acceptable in lieu of 4874459 by engineering. ATF+4 is a high-quality oil but has a thinner viscosity than the original 75W85 lube. This can be beneficial for economy and heat generation. In high heat high load conditions this can be undesirable as the oil is not thick enough to maintain a film thickness large enough to prevent wear. ATF+4 contains low levels of EP additives and poses low risk to synchronizer wear. This oil is perfectly acceptable for trucks that are not worked hard regularly.
Option 2: Red Line MT-85 75W85 GL4
Red Line markets this oil as a replacement for Mopar Part# 4874459. It has a slightly lower viscosity than the original oil. This oil is the closest aftermarket replacement to the original oil.
Option 3: Amsoil MTG 75W90 GL4
Amsoil markets this oil as a replacement for Mopar Part# 4874459. It has a slightly higher viscosity than the original oil. You may notice a slight decrease in fuel economy and higher oil temperatures using this oil. This higher viscosity will provide a higher film strength at operating temperature. This is beneficial for trucks that are typically loaded down and worked hard.
Option 4: Valvoline Synchromesh GL4
This is a generic manual transmission oil. It has a lower viscosity than the original oil. This can be beneficial for economy and heat generation. This oil is perfectly acceptable for trucks that are not worked hard regularly.
Option 5: Royal Purple Synchromax GL4
This is a generic manual transmission oil. It has a lower viscosity than the original oil and is very similar in viscosity to ATF+4. This can be beneficial for economy and heat generation. This oil is perfectly acceptable for trucks that are not worked hard regularly.
Option 6: Mobil Delvac 1 SAE 50
This oil is generally used in heavy duty over the road trucks. It has a very high viscosity compared to the original oil. You will notice a decrease in fuel economy and higher oil temperatures when using this oil. This oil will offer the highest level of protection and is recommended for trucks that are always loaded and worked hard.
Remember there are always other oil options that may fit you need and budget. We have only covered oils that are regularly discussed in forums.
Below is a chart comparing the viscosities of the above oils. The vertical axis is the viscosity at 40C (104F) and the horizontal axis is the viscosity at 100C (212F). The multicolored line is the range of each SAE gear oil weight grade.