Since the beginning, I've always run Dexron ATF in my tranny. Been thinking lately... I know the fluid must be a GL4. So, instead of running the heavy 75W-80, or the ATF, what would the effect be of running, say a straight 30 or 40 weight engine oil be? The only reason the I'm considering a change is that my tranny, being an 18 spline and slick shifted, is kinda noisy on the street. Lately, I've been doing more and more street driving. Thus the noise issue.(or non issue) Thoughts???

Try Redline MTL

When I was a Honda tech back in the mid 80's they ran straight 30 weight in all their manual boxes and they shifted like butter and lasted half a million miles Never tried it in my boxes but I do run one quart of 30 weight motor oil mixed in with the mineral based 75-90. I've see a few cluster gears flaked where the rollers ride from ATF.
I ran the Redline in my Richmond when it still had syncros and I didn't like the way it power shifted.
Gus

Try using the new Pennzoil manual tranny lube

I switched to the Redline oil this year, and am not impressed with it. Noticeably harder to shift. I only have 6 passes on it though. If it doesn’t get better next time out, I’m dumping it, and going back to Mobil 1 synthetic.

Remember that gear oil weights are not the same as the motor oil weights...
https://www.thelubricantstore.com/understanding-the-viscosity-grade-chart

My Vote is 30 wgt with some STP.

Well, I drained the ATF and Installed three quarts of conventional 30W engine oil. I just did a road test and it is much quieter than before. All 18 splines are a little louder than the 23 splines. It's something that has to be lived with. With that, I am a big fan of STP. I religiously use it in my engines. I don't see any reason to use it in a manual transmission though? Thanks for all the replies and suggestions. Really appreciate it, especially that chart.

Adds to the lubricity (slippery- ness) of the oil. The 30wgt protects the tooth to tooth and bearing lube and the STP helps the sliders also gives it a little more cling.

Here's an interesting talk about lube for manual transmissions.
Jump to 2.28 for the start of his discussion about motor oil for manual transmissions.
https://www.youtube.com/watch?v=m_toEAYSOB0

Very Cool! Interesting!!! Although there are many other oil manufacturer's that tell their ZDDP content.

So going straight Mobile 1 20-50 which states it has ZDDP for older cars might be just right?

I would probably use something like 10-30 or so high mileage. It will give you a 85W when warm/hot. Straight GL-4 lube is getting harder to find. Everything seems to be GL-5 for rear ends.

hello,
at one time Chrysler had a service bulletin "if trans is noisy drain and refill with 30W oil, may take more effort to shift"
gary

Every 833 that I've taken apart that was run on ATF need the cluster pin replaced , you could catch a fingernail on the lip where the rollers ran.

FACT. I have one in the shop right now that had ATF in it and it needs a new main shaft because it is flaking where the main shaft goes into the input. Seen this so many times over the years it’s just dumb to use ATF in a gear box.

ATF and running the box dry are the only things that cause that issue. If you live where it gets cold, any decent synthetic gear oil will be far better than ATF.

Bellhousing misalignment will also cause flaking on the input. I had to redo a few for the early hemi swap/adaptor plate guys, and the lakewood bells seem to be real bad. I just rebuilt a trans for a guy, always atf 90,000 miles on it, countershaft pin looked like new, if fact everything looked like new. ATF is not always the problem on street driven cars, but the trans needs to be full, and atf likes to leak. I just did the one in my 340 cuda, 68,000 miles, it was mint, but leaking. Sometimes people bring me a trans and it has 2 qts of dirty oil in it, and plenty of wear, gee I wonder why. We need to admit with high horsepower and slicks that lower pin design is marginal, even worse low on oil. I have seen that worn no matter what is in them, but I also do not know what was run prior. We also need to remember how all the cool guys drive around in third, on off the throttle to sound cool, that puts the most wear on that lower front of the pin, just like the overdrives that are always wiped out right there. I used to run a 30wt motor oil/atf blend and they shift really good, synthetics never worked well for me or others I know. Gl4 mineral gear lube also works well. We also used 2qts of 10w30 in our automatics with j converters and never an issue. Your results may vary, but a properly rebuilt a-833 full of oil and properly aligned is problem free.

i remember years and years ago, a quart or two of motor oil in an automatic was recommended in a magazine article, perhaps by marvin ripes ?
maybe back in the early, early 80's ?