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Manual Transmission Gear Oil ..... use the right stuff !!

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this is a LESSON LEARNED type of post.

A couple of days ago I changed the gear oil in the '74 tii's transmission. The owner's manual says, "80W non-hypoid gear oil."

But yesterday bourkeco warned me to check the GL rating, that I wanted GL-4, not GL-5. GL-5 stuff contains sulfur which is harmful to the brass parts (synchros) in the transmission. All these years I had never heard of such ratings, and no idea how many years I might have used a GL-5.

And sure enough, I had just put GL-5 into the transmission.

So I did some info searching, here's a couple of threads .....



So today I bought Redline MTL and will be getting that into the transmission today.




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Carl - glad you "found it"

i've been preaching GL4 forever - same issue with vintage

(Pre-1980ish BMW motorcycles)

so again, for new-to-GL4 info people - you can get

GL4 spec from a BMWDEALER also


back pre-2002faq when i lived at roadfly

i posted:

BMW 1600 - 2002 4 & 5 speed trans oil GL-4 only!

The BMW specification for your transmission is GL-4 80W

per BMW Repair Manual and BMW Owners manual

BMW - MINI now offers a 5 Litre (1.32 U.S Gal.) bottle available from any BMW - MINI dealer. Part Number , suggested list price $119.00,


This is also the gear oil for MINI Getrag 252-5 speed and 285-6 speed

produced from 07/2004, see MINI SI M 00 02 02 March 2005

other available GL-4 gear oil:



All modern gear oil available today in local retail outlets is GL-5 grade. GL-5 grade gear oil contains sulphur compounds that attack brass and bronze. BMW transmissions and final drives made before 1992 contain bushings and thrust washers made of these metals. Any brand of gear oil that is GL-5 rated will attack bushings and thrust washers.

Pennzoil Specialty Gear Oil For Applications Requiring GL-4 Performance:

PENNZOIL® GEARPLUS® SAE 80W-90 GL-4 GEAR LUBRICANT is specially formulated to meet the unique requirements of API GL-4 service in manual transmissions and transaxles of passenger cars, light trucks and SUV’s. PENNZOIL® GEARPLUS® SAE 80W-90 GL-4 GEAR LUBRICANT is blended from select base stocks and additives that yield a lubricant of exacting quality. This product is compounded utilizing specialized extreme pressure chemistry that provides protection of gears and bearings without being corrosive to bronze components. It contains foam suppressants to prevent wear, which can be caused by lubricant foaming and aeration. Effective rust and corrosion inhibitors protect metal surfaces from the effects of water contamination from condensation and the operating environment. PENNZOIL® GEARPLUS® SAE 80W-90 GL-4 GEAR LUBRICANT resists the formation of varnish deposits on synchronizer components, thus assuring long, trouble-free operation.

PENNZOIL® GEARPLUS® SAE 80W-90 GL-4 gives the full protection of an SAE 90 under hard driving conditions while providing the flow characteristics of SAE 80W that are necessary during cold weather start-ups. This offers maximum lubrication and gear protection over a wide temperature range.

PENNZOIL® GEARPLUS® SAE 80W-90 GL-4 GEAR LUBRICANT is designed for manual transmissions and transaxles of both American and import vehicles. It is formulated to be compatible with “yellow metals”, such as bronze, found in synchronizers and other transmission componenets.


• Meets the performance requirements of API GL-4 Service

• Will not cause glazing of synchronizer components

• Excellent thermal stability

• Outstanding extreme pressure and antiwear protection

• Prevents rust and corrosion

• Inhibits foaming and aeration

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I took apart a 4- speed that had had the wrong oil in it,

and was shocked- the oil really DOES dissolve the synchros!

They were 'gummy' and the one in gear was stuck to the cone.

I would never have guessed....


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GL-5 is not necessarily backward-compatible in synchro-mesh transmissions which are designed for a GL-4 oil: GL-5 has a lower coefficient of friction due to the higher concentration of EP additives over GL-4, and thus synchros can not engage as effectively. Also, transmissions which explicitly call for GL-4 oil may have been designed around this lower concentration of EP additives and thus may contain yellow metal parts which GL-5 will corrode.

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Yarp- that was it. The synchos were, in fact, corroded.


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