travisc21

Cant seem to get a straight answer about what kind of tranny fluid i need for my truck. Its a 91 f-150 5.0 L XLT Lariat 4x4 with a manual transmission. When i bought the truck it didnt come with the owners manual so i cant look it up.

So, anybody know what kind of tranny fluid i need?

fatboi07

A quick looksey on all-data tells me that make/model/year came with 5 speeds and 4 speeds. Which is yours?

Just call me Sean

iTrader: (1)

1992 and earlier 4 speed- SAE 80W gear oil
1993 on 4 speed and all 5 speed- Dextron/Mercon III

fatboi07

Damnit sean beat me to it lol. I was waiting for the 5vs4 reply. But yeah what he said

wormshoe

I can only tell you what works for me. I used to run the automatic trans fluid as recommended by Ford. During one engine oil change, I decided to drain and refill my 5 spd. with the same oil I put inot the engine....Mobil One 5W-40. It shifted smoother than ever...and it still does years later. I wouldn't consider using anything else. Just my two cents.....

wormshoe

Just call me Sean

iTrader: (1)

Ok Mr. Mad Scientist, if you like it then you like it. Not all of us like to do potentially costly experiments on our trucks though. It would be best to use what's recommended.

fatboi07

I know some of its got to do with the higher viscocity being able to drag away particulate (blocker ring matter etc.) at higher temps, as well as being able to keep that viscocity at higher temps.

mitchalvin

If you use heavier oil you have the potential of not lubing the smaller bearings and other surfaces, remember these only have splash lubrication. I believe that ATF is around 2w or something. Using a synthetic ATF would probably help with smoother shifting without running the risk of eating transmission parts.

travisc21

This may be a dumb question but it has 4 gears plus overdrive. Is that 4 speed or 5 speed?

wormshoe

I was told by a spokesperson at Mobil that Ford initially went to auto trans fluid because having a lower viscosity, it actually helped fuel mileage. Moving gears through heavier lube actually takes more power.

The same for rear axle lubricants. Years ago, everyone recommended heavier viscosity lube oil....now they are using thinner stuff because of the mileage issue. My Mazda transmission shifts smoother now than it ever has and I have had the truck since April of '89.

You can't go wrong by following Ford's recommendations however. Use the auto trans fluid in your 5 spd. and feel good about it.