Why does my Isuzu specify engine oil for use in manual transmission? Can I use 75w80 gl-4 instead?

[leoblack9]

Hello ladies and gents,

My isuzu d-max has a 5 speed manual transmission. But when I look at the manual it specifies a very specific Besco 5w30 transaxle fluid OR 15w40 CI-4 engine oil (Delo 400 15w40, Mobil 1 delvac mx 15w40 etc.) and researching around tells me that Isuzu likes to put engine oil in their manual gearboxes.

Wouldn't a gl-4 fluid be better suited for this function? I'm curious to try 75w80 gl4 in my light diesel truck but would want to ask around first if its a good idea. According to other Isuzu forums, their recommendations are to use 5w30 C3 or 10w30 engine oils but I wonder if the additives for the engine would be good enough as opposed to a purpose built MTF/MTL fluid.

 

[ripcord]

Using 30 weight engine oil has been common in a lot of Japanese manual transmissions over the years. I would follow Isuzu's recommendations. SAE 30 weight is equal in viscosity to 80w or 85w gear oil as they are just rated with different scales.

Transmissions do not have the hypoid type gears that differentials do, so they don't require the same extreme pressure protection. In a transmission, the main job of the oil is to lubricate the bearings.

 

[weasley]

Using 30 weight engine oil has been common in a lot of Japanese manual transmissions over the years. I would follow Isuzu's recommendations. SAE 30 weight is equal in viscosity to 80w or 85w gear oil as they are just rated with different scales.

Transmissions do not have the hypoid type gears that differentials do, so they don't require the same extreme pressure protection. In a transmission, the main job of the oil is to lubricate the bearings.

True - my old Honda specified 10W-40 engine oil for the manual transmission. Just to add though, one significant factor for MTFs is friction control of the synchromesh materials. And whilst the gears are not hypoid, they are helical so there is still a roll/slide action between the teeth needing some EP protection.

 

[Chris142]

My 87 f250 uses motor oil in it's transmission. Works fine.

 

[burbguy82]

I do not see 15w40 in the list on your post, but......If it would allow 15w40, that is what I would go with. As I understand it, the OCI is lessened when you use engine oil as opposed to gear oil, due to shear stability of the gear oil being greater. Gear oil is "tougher" than motor oil..............in a gear box.

My manual specs gl3-4, or 10w30 motor oil. I use a

Same thing in the Tcase.

Not sure how you drive, or your applications, but I would use engine oil for 5k-7K under harsh conditions, and drain.

Yeah it is a bunch of oil, but your drivetrain will be in great shape for years to come.

 

[Escape08]

My 1984 Isuzu P/Up use 10W-30 in a 4 speed M/T

 

[ctechbob]

If you do use a gear oil, make sure it is safe for yellow metals, your synchros will thank you.

 

[CharBaby]

I would follow your ISUZU's owner's manual for this.
Yeah, many older Honda MT's required 10W30 or 10W40 for their manual trannies. But that's when oil had a different additive pkg.
I don't think that's what Honda manual trannies would require today.

 

[ARB1977]

Same goes true with my Tundra. The transfer case requires ATF…not gear oil.

 

[paulo57509]

I remember finding ATF in a '68 Chrysler Newport 4-speed.

The Legend specs SE/SF, SAE 30, 10w-30, 10w-40, 20w-40. I use RedLine MTL.

 

[someonesomewhere]

Same goes true with my Tundra. The transfer case requires ATF…not gear oil.

Ford's ubiquitous M5OD truck transmission is the same. Used in Rangers and F150s. Specs Dex III/Merc. Or did until 2002, anyway (my newest that I've owned). Smoother function with Penn Synchromesh, though.

 

[johnmyster]

Ford's ubiquitous M5OD truck transmission is the same. Used in Rangers and F150s. Specs Dex III/Merc. Or did until 2002, anyway (my newest that I've owned). Smoother function with Penn Synchromesh, though.

And the Thunderbird SC, 1989-1995. The car of my youth. But the M5OD was actually a Mazda design. IIRC, dad and I purchased and fixed up 7 of them from auction at various times. A few were long term keepers. A few autos, a few manuals. We took more than one of them apart to replace weak 2nd and/or 3rd gear synchros. One of them seemed to do better with a stout ATF (B&M trick shift I believe) for a few miles.

As far as transfer cases go, some of them live just fine on motor oil, atf, or gear lube. Unless they have a wet clutch in them, they're pretty insensitive. It's just a mechanical box with some bearings, gears, and chains that needs to be lubricated...but they usually benefit from some anti-foam properties anytime the chain is spinning.

 

[eljefino]

The New Venture trans in my 91 Dakota called for 10w30 and it felt FANTASTIC, especially for an economy spec truck.

 

[alarmguy]

True - my old Honda specified 10W-40 engine oil for the manual transmission. Just to add though, one significant factor for MTFs is friction control of the synchromesh materials. And whilst the gears are not hypoid, they are helical so there is still a roll/slide action between the teeth needing some EP protection.

Same here, decades back my five speed, Honda Accord hatchback and my CRX was 10/40
Reason for everything and I would use the recommended oil and not second guess by using gear oil where it’s not called for

 

[weasley]

Same here, decades back my five speed, Honda Accord hatchback and my CRX was 10/40
Reason for everything and I would use the recommended oil and not second guess by using gear oil where it’s not called for

Mine was a CRX too! An EE8 (VTEC model). I actually tried a high spec transmission oil in it and it was dreadful, so went back to a 10W-40 synthetic blend which was immediately better.

 

[alarmguy]

Mine was a CRX too! An EE8 (VTEC model). I actually tried a high spec transmission oil in it and it was dreadful, so went back to a 10W-40 synthetic blend which was immediately better.

Gosh I loved that car. Bought the CRX new around 1985 ish I think. It was just so "analog" and fun to drive, loved the hatchback, everything.
The 5 speed (or was it 6?) manual shifted like butter. It had soul.

Maybe because I am older but now, as an example. My wife's new 2025 Equinox, freaking car is loaded with every electronic and it's neat since I love electronics, Really amazing but everything is very distracting at the same time.

When I get in the Equinox, it's like everything else in my life, electronic, nothing analog about the car, NOTHING. I feel like when I get in the car it's an extension of everything else in my life, digital electronics. From my desktop computer, to my laptop, to my iPhone, to my Apple Watch to my TV, my kitchen to yes and now to the car. Oh, well, I guess if that was my only problem in life I would be in great shape... but WOW I loved that CRX. It was just so unique. Simple to work on too, I loved adjusting the valves, changing the oil, changing the transmission oil, tuning it up...

 

[TimeFlies]

Many manual transmissions use engine oil, I've owned 2 vehicles in the past that did.

Gosh I loved that car. Bought the CRX new around 1985 ish I think. It was just so "analog" and fun to drive, loved the hatchback, everything.
The 5 speed (or was it 6?) manual shifted like butter. It had soul.

Maybe because I am older but now, as an example. My wife's new 2025 Equinox, freaking car is loaded with every electronic and it's neat since I love electronics, Really amazing but everything is very distracting at the same time.

When I get in the Equinox, it's like everything else in my life, electronic, nothing analog about the car, NOTHING. I feel like when I get in the car it's an extension of everything else in my life, digital electronics. From my desktop computer, to my laptop, to my iPhone, to my Apple Watch to my TV, my kitchen to yes and now to the car. Oh, well, I guess if that was my only problem in life I would be in great shape... but WOW I loved that CRX. It was just so unique. Simple to work on too, I loved adjusting the valves, changing the oil, changing the transmission oil, tuning it up...

They were a 5 speed.
Awesome cars!!!

 

[nthach]

Honda called for 10W-30 motor oil until they introduced their MTF. Mazda and Ford called for ATF.

If you must use motor oil, a diesel HDEO meeting Allison C-4/TES-439 or JASO MA/MA2 specs would be my pick. PCMOs have friction modifiers that might not play nice with synchros. I would think Red Line MTL would be fine, trucks with Eaton or Dana manual transmissions call for a SAE40/50 transmission fluid.

 

[Pablo]

I would get an MTF in the 9.5-11 cSt range @ 100°C. There are options in the Philippines.

 

[kschachn]

I would get an MTF in the 9.5-11 cSt range @ 100°C. There are options in the Philippines.

Agreed. In my experience a motor oil is a poor MTF. It “worked” in my old Honda but not nearly like a proper fluid.