Gearbox Oil Choices

Ok, so I'm very new here but I did just run the same question by a few 'experts' and digested a mountain of oil threads. I'm going to use Redline MT 75w90, a GL-4 oil. I think the key is not the brand or even the weight but GL-4 vs GL-5 or 4/5 oil.

 

FWIW:
I called redline tech line a long time ago to inquire about trans oils. I was told for a 308, "DO NOT use anything but GL-5 rated oil". I recall the shock loading was the property to be most concerned about. Also, I recall that it's bearings that are most affected as opposed to gear faces and I thought this was unintuitive because bearings either require thin film protection for high loads, or heat removal properties for high speed (dental drill etc) but shock loading - interesting? Also they were opposed to putting MTL in a Ferrari trans. I do use redline in my cone synchro trans and SWEPCO in Porsche thrust synchro (Dino) boxes.

Anyway, I was glad they had a solid opinion which I'd expect to come from real testing & data and would recommend calling them directly.

 

My gear lube of choice in manual transmission and differential is Mobile 1 synthetic LS 75w-90. You can buy at your local auto parts store. My company makes gearboxes. A good synthetic gear lube seems to run much cleaner than a comparable petroleum gear lube. We have 1000's of testing hrs on both types. Cost seems to be the biggest factors for OE's choosing petroleum. There is nothing wrong with petroleum gear lube it will function fine a quality synthetic is my personal choice. NOTE: DO NOT USE EP 80-90 in your manual transmissions the EP additive is for extra pressure and will corrode your expensive/unabtainium bronze synchros. The EP additive is fine and is specifically for your differential.

 

I don't know. Redline is a good company. Call them tell them the oil you used they will tell you. I haven't used their products since the sportscar racing days.

 

You made the right choices for your model's configuration. The EP (sulferus) additive is in the GL-5 gear oils as the hypoid gears have some sliding action under high load in their meshing.

 

RETRACTION/CORRECTION Steve is correct. I was of the understanding that the GL5 additives in the 75-90 Mobile gear lube was compatible with bronze. The spec sheet says so. Just got off of the phone with the Industrial Mobil Tech and he said NO. Guess I won't be using it in my manual transmissions any more!

 

Thanks for the info guys.

 

Your 250/82 OM specifies a GL-5 gear oil (AGIP ROTRA MP SAE 80W90: http://www.uleiauto.ro/phpeuri/oiluri/agip/ROTRA%20MP%2080W-90.pdf) for both the gearbox and the differential -- so no worries there. And using a GL-4 in the gearbox is no sin -- I've noted on other F models, with separate lubricant reservoirs like yours, that they use a slightly less viscous GL-4 in the gearbox and GL-5 in the diff (but that was before the days of multi-range viscosities so using a straight 90W GL-5 may have not shifted as well as a 80W90 or 75W90 GL-5 would today).

 

If you mean on your 365GT4 2+2, I don't think that you have to worry about it as your 75/73 OM shows using the same GL-5 gear oil (Shell Spirax HD 90: http://aglayne.com/wp-content/uploads/2010/11/Spirax-HD-80w90.pdf) in the gearbox and diff. Your syncros might be a sintered steel alloy rather than a bronze alloy (but I don't know for sure). You might try a GL-4 in the gearbox next time to see if you notice any difference in the shifting feel -- just a thought...

 

I have seen a syncro and it didn't look bronze in color.

 

Does anyone have any new thoughts on the best oil for a 365GT/4 (5spd transmission)
I have Castro Syntrax universal plus 75-90w recently added and it's a little bit notchy than I would have liked
https://www.castrol.com/en_nz/new-zealand/products/cars/axle-universal-oils/universal-plus-75w-90.html
It seems to tick all the boxes. GL-4/GL-5 etc

Thanks for any thoughts

 

I have tried several different gear oils in my transmission. I have had the best results with a GL4 not a GL4/GL5 oil. I currently have Amsoil 75-90 GL4.

 

GL4/5 is mostly marketing hype. It is not GL4. GL5 is appropriate for differentials and is counter productive to the needs of syncros, brass or not. Get GL4.

 

I could tell an improvement with GL4.

 

Kernite oil is(/was) nice on gearboxes with fragile synchro, it (for sure) gives a better grip and (presumably) reduces wear. This is what the Subaru guys were using here in France. This oil is not used a lot any-more, as the rules do impose stock oil now on many classes, but it is still a good choice for vintage cars.

Side note: obviously not tested on my 400i auto, but made quite a difference on my refurbished R5 alpine gearbox: oil additives do provide similar results on worn synchros, but Kernite feels better when the oil is hot.

Kernite now belongs to NCH.

 

I use "Castrol Syntrans Transaxle 75W90". Works perfect…..