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.