I own a stock 1979 Dodge pickup /6. Manual 3 spd. trans. with column shift. I think these trucks could use either 80W-90 gear oil or Dexron transmission fluid.It will only be used in the summer.Should I use Redline, Royal Purple, Amszoil,or something else? What does your slant6 like? THANKS.

The NP435 3-speed trans in your truck wants either GL-5 gear lube as follows:

Above 90°F: 140W
Above -10°F: 90W
Below -10°F: 80W

or high-quality engine oil, as follows:

Above 32°F: Straight 50-weight.
Below 32°F: Straight 30-weight.

If you want to pamper your truck, you may want to try Mobil-1 75w-140 synthetic gear lube in it. Should be easy to find locally. There is no need or benefit to seeking out exotic brands (e.g. Royal Purple) or drinking the Scamsoil kool-aid.

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Too many people who were born on third base actually believe they've hit a triple.

NP435 is a four speed with first being granny, top load, top shifter.....you do not want to use Dexron in it if you have one, 90w or higher in most
app's.

Three speeds---- top or side load trans with side shift arms
almost always a column shift.
My guess is that a 3 on the tree would be a NP250 or the like...
and should use as you posted 80W-90 gear oil or Dexron transmission fluid.

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62 D100 225 3sp lwb
64 D100 A318 727pb custom lwb
66 D100 A318 4sp lwb
68 D100 B383 727 swb
65 Belveder A318 727 4d
65 Dart GT LA273 2bbl 904
73 Scamp 225 2bbl 4spOD
Old iron or no iron

Using engine oil is OK, but it has detergents and additives to help remove deposits in the engine, if left for long periods of time they turn acidic and can 'chew' on your brass synchros (and the aluminum case if you have one of the late 'other' manual trannies).

There still are brands of 'transmission' oil (like the old GL-4), GL-5 is usully 'Hypoid' which is great for rear-ends, but can also cause the tranns to 'skip' a shift due to it's properties. Most places 90 wt is fine (wintertime the tranny will be a bit stiff to shift in winter, but it warms up after 3-4 blocks).... ATF was recommended by mopar and works OK, but it's thinner and has a tendency to leak faster if the seals are bad in the tranny, and doesn't 'stick' to the gearing as well (like 90wt syrup)...ATF would be a good choice for drag racing, 90wt has been my family's choice for 3 generations now).

-D.Idiot