You ask a very good question........ I assure you the 70s FSM..... all of them say to use GL4 Gear Lube. I even went and researched and quoted every Jeep FSM, chapter, page, and verbage...... and they almost all said to use GL4 in the Jeep Trans and Transfer case. The exception would be the 1982 FSM and said GL5 and JEEP had a full line, accross the board, full recall of all trans fluids Oct. 1982 to replace the GL5 because so many transmissions were failing in all models.... it was a big mess.
There are a few.... very few.... GL5s that will work but not work as well as GL4.... they have different properties and are NOT interchangeable.
Originally Posted by Fjguercio
Here is a POST from me a few years ago.... I just copied and pasted here so hope nobody minds. YOU WILL FIND NOTHING BUT GL5 GEAR LUBE ON THE STORE SHELVES, NOTHING BUT.... The GL5 GEAR LUBE is the wrong fluid for old JEEP CJ Manual Transmissions and Transfer Cases, can also cause the SYNCROS to start rattle/noise and not work well or shift well. GL5 HAS THE WRONG.... LUBRICATION PROPERTIES.... AND.... IT IS CORROSIVE TO YELLOW METALS USED IN MANUAL TRANSMISSIONS. Some have posted, many posted, they could hardly shift after some time. Changed to GL4 and got better and quieter. I have done this dozens of times with great results.
So Please copy this to your hard drive and spread the word. This could stop a lot of JEEP MANUAL TRANS FROM FAILURE AND REBUILD BEFORE THEIR TIME. TRY IT YOU WILL SEE.................
The confusion with the AMSOIL product its rated as oil viscosity and not Gear Lube viscosity. The TWO OIL VISCOSITY SCALES cross or equal at these weights…..AMSOIL MTF is what I am going to change to since I do not WATER CROSS so the installed life will be of benefit. I currently have the Napa GL4 installed and very pleased with the correction of operation and is getting even better over time…. Almost no syncro rattle now
THE AMSOIL MARINE GEAR LUBE CAN ALSO BE USED FOR......... GL4 GEAR LUBE, it meets the GL4 spec, it can absorb more water before oil failure. SO... for those of you that play in the water alot that may be a good choice.
I have spoke to Amsoil Tech Support and the Gear Lube Engineer..... This is confusing to MARKET. Using Gear Lube Weight for gear lubes and OIL LUBE WEIGHT for the MTF product. They are changing their lit and web site to use Gear Lube Weight for MTF / GL4 Product. Should be done sometime next year. A Chem Eng In OIL here on JeepForum and I had a WEB Discussion on this in detail.... seems the Oil Weight and Gear Lube Weight cross here at these numbers. I’ts just confusing.
This info has been posted lots….. and many times on the forums. MOST of the Jeep Field Service Manuals say use GL4….. the only place Jeep messed it up was 1982 they said GL5 and had INDUSTRY RECALL ON ALL MODELS and changed the gear lubes and transmissions left and right…. Or very often.
The Amsoil White paper on gear lubes is a very good read.
The Novak Web site does have a little blurb on gear lubes/trans and its very confusing, not well written, and its leading many down the wrong path. I wish Novak would rewrite and update their info.
Gl4 and GL5 are not the same and not interchangeable…….. their applications are different…. Their properties are different….
It your syncors rattling. The gear lube on the shelves is GL5 and is a product made for the pumpkins of these big large towing capacity tow rigs. The GL5 has the wrong lubrication properties and is corrosive to the yellow metals in manual transmisisons. All the BorgWarner Transmissions in the older Jeeps should use Gear Lube.
The RIGHT GEAR LUBE is GL4...... GL4...... GL4.... Napa Sells it, call them and ask them to order you 1-2 gallons and its like $25 a gallon.... was $15/gal when I first started posting... not the price has almost doubled..... After 50 to 100 miles it will start shifting better and should start to get quite. My T18 did the same thing after I put the Gear Lube from the shelf.... it was GL5 and started shifting harder the the syncros starting rattle. Now almost silent and shift much better and its 32 years old.
Alternaitves Amsoil Syncromesh is a GL4 or Mobil 1 were both rated very high and and some of the least expensive on the list.
Bromgarrds Farm Stores, IA, NE, Co, Midwest area also have a GL4 gear lube. Sell 2.5 gal container for like $27.
I have posted this up many times, many folks amazed with results. Frankly I am SHOCKED another fluid bad for our Little Jeeps ALL OVER THE SHELVES. I HAVE LOOKED YOU WILL ONLY FIND GL5 ON THE SHELVES. Good for your pumkins but not for the trans/transfer. Also add a magnet to both drain plugs to catch the metal particle. I also took a "stick magnet" and rubbed the bottom of trans/trans and got more out that way.
NAPA
SAE 85w90 Gear Oil GL4 (also meets GL3)
Part #SL24239
Mfg By Sta-Lube, sibisdary of CRC Industries
Bar Code 72213-24239
$25/gallon, phone order take 1-2 days to arrive, then pick it up, easy
I would also consider Amsoil MTF Syncromesh GL4 or the Mobil 1 Gear Lubes they were tops in performance and lower in cost that most. $8-10/quart.
PS
Use GL5 Gear Lube in your Pumpkins and follow any recommendations for limited slip or locker additives.
APPLICATIONS For GL4 and Amsoil MTF Gear Lube their GL4
Recommended for automotive and light-truck applications that require synchromesh transmission fluid. Applications include manual transmissions and transaxles such as New Venture NV T350, NV 1500, NV 2550, NV 3500, NV 3550, NV 5600, and Tremec T4, T5, T18, T56, T176, TKO500, TKO600, TR 3450 and TR 3550. Replaces MTF-94 fluid for Land Rover, MG, and Mini Cooper. Replaces Honda Genuine MTF fluid for manual transaxles and Texaco MTX fluid. Not for use in engines, hypoid rear axles, limited-slip or wet clutch applications.
T5 .....Dexron III / Mercon, Mercon V use.....ATF.......
BUT....(except T5 from 1985 to 1991 use GL4 Gear Lube)