Red Line MTL 75W80 GL-4 Manual Gear Oil Case of 12 Quarts

If your NV3500 Ram or GM truck, an NV5600 diesel, a Jeep with an NV1500 or NV3550, a Land Rover LT77S or a Honda or BMW manual gearbox crunches into second when it is cold, or the lever feels notchy until the truck warms up, the gear oil is the first thing to look at. This is a full case of twelve one-quart bottles of Red Line MTL 75W80 GL-4, part number 50204, the synthetic manual transmission oil Red Line lists for exactly those boxes. A case is the sensible way to buy it for a shop, a fleet of trucks, or an owner who wants a fill and a spare change on the shelf.

What MTL does in a synchromesh manual gearbox

A manual transmission is not a differential. The gears are mostly straight-cut helical gears turning on a shaft, and every shift depends on a brass or bronze synchronizer ring rubbing against a cone on the gear until the speeds match. That ring needs friction. Many synthetic gear oils are too slippery for the ring to grab, and the shift turns long and floaty. Hypoid GL-5 axle oils have a second problem: Red Line notes they carry reactive sulfur additives that attack yellow metal, so the synchro rings wear and glaze. MTL is a GL-4 oil without those reactive sulfurs, friction-balanced so the synchro bites, and thin enough as a 75W80 to keep cold-morning shifts from going stiff. Red Line rates it as meeting the viscosity requirements of 75W and 80W gear oils and of SAE 30, 10W30 and 5W30 motor oils, which is why it covers boxes that originally called for engine oil or a synchromesh fluid.

Symptoms MTL is meant to cure

• Grinding or a hard block going into second or third, worst on the first few cold shifts
• Notchy lever feel that smooths out after twenty minutes of driving
• Rough shifts after someone filled the box with GL-5 axle oil
• Old, dark oil with a burnt smell or fine brass glitter on the drain plug magnet
• Excess gear whine at cruise from oil that has thinned out with age

Oil can not fix a ring that is already worn smooth or a broken shift fork. If a gear still grinds with fresh oil and a fully released clutch, the synchro itself is the problem.

How to confirm the oil is the issue before you buy

Pull the fill plug first, not the drain, so you know you can refill it. Check the level at the bottom of the fill hole and look at what drains out. Metal paste or broken teeth mean the box needs work. Next, rule out the clutch: a dragging clutch from a weak hydraulic release makes any gearbox grind, and on the NV3500 the pre-bled clutch master and slave cylinder is a common fix. For a sense of what manual repairs cost compared with a fluid change, read our guide on automatic vs. manual transmission repair cost.

Fitment

Red Line lists MTL as popular for BMW, Honda and Acura, Ford Focus, late-model Mini Cooper, Land Rover and Range Rover LT77S, and the Dodge and Jeep NV1500, NV3500, NV3550 and NV5600 truck transmissions. It is offered as a replacement for GM Synchromesh fluid and Chrysler spec MS-9224, and is recommended for GL-1, GL-3 and GL-4 applications. Fill specs vary by year and vehicle, so confirm in your owner's manual or service data that your box calls for a GL-4 or synchromesh-type oil before filling. MTL is not for use in a differential with hypoid gears, and it is not for a transaxle that shares oil with a hypoid final drive or calls for ATF. If you are unsure, send us the year and VIN and we will check. More manual parts are under our fluids category.

Install notes

• Loosen the fill plug before the drain plug, drain warm, and clean the magnet.
• Fill until oil just starts to weep from the fill hole with the vehicle level.
• Use new crush washers or sealant on the plugs as your service data specifies.
• MTL mixes with petroleum and other synthetic gear oils, but a full drain gives the best shift feel.
• If the box still grinds after the change, plan on synchros. On a GM NV3500 a worn shifter assembly causes sloppy engagement too, and a worn NV3500 clutch kit replacement fixes drag that oil cannot.

Is Red Line MTL the right oil for an NV3500?

Red Line lists the Dodge and Jeep NV3500 among MTL's popular applications, and it is sold as a replacement for GM Synchromesh fluid, the fill used in many GM NV3500 trucks. Confirm your owner's manual calls for a synchromesh or GL-4 fluid first.

Can I use MTL in my rear differential?

No. Red Line states MTL is not for use in differentials with hypoid gears. Axles need a GL-5 hypoid gear oil such as Red Line 75W90.

Why not just use a GL-5 75W90 in the gearbox?

Most GL-5 oils carry reactive sulfur additives that Red Line says damage brass synchronizer rings, and they are often too slippery for the ring to grab. The result is grinding and notchy shifts.

How many quarts are in this case and is it enough?

The case holds twelve one-quart bottles, part number 50204 each. That is enough for a fill and spares on most light-duty manual gearboxes; check your vehicle's capacity in the service data.

Can I mix MTL with the oil already in the transmission?

Red Line says MTL is compatible with petroleum and other synthetic oils, so topping off is safe. A full drain and refill gives the best shift improvement.

What is the difference between MTL and MT-90?

MTL is the lighter 75W80 grade for boxes that call for synchromesh fluid, 5W30 or a 75W80. Red Line makes heavier manual transmission grades such as MT-90 for boxes that specify a heavier gear oil; match the grade to your service data.

• Automatic vs. Manual Transmission Repair Cost: What to Actually Expect