Click on image for the lowdown. 
====
| | | Forums66 Topics128,110 Posts1,056,246 Members48,437 | | Most Online14,466
Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
| | THANKS!
For helping keep this great
old truck web site
on the Internet for over 30 years! and not clogged down
with Ads!  Again, thanks!
| | | | Joined: Apr 2013 Posts: 82 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2013 Posts: 82 | I recently obtained a 1951 gmc long bed. I would like to change the differential oil and transmission oil. I would like to know what weight oils to put in them? Thanks for you help in advance. | | | | | Joined: Oct 2006 Posts: 9,671 'Bolter | | 'Bolter Joined: Oct 2006 Posts: 9,671 | Just head to your FLAPS or local farm store and pick up 75w90
and that will take care of all your transmissing and axle needs.
That's all that is used in many over the road fleet trucks. I'd
stay away from the more expensive synthetics especially if it's an SM420 transmission.
Denny G
Last edited by Denny Graham; 04/27/2013 11:05 AM.
Denny G
Sandwich, IL
| | | | | Joined: Apr 2013 Posts: 82 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2013 Posts: 82 | Thanks for the advice. Now, Isee the drain plug on the transmission, but where is the fill plug. By the way it is a 4 speed. | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | | | | | | Joined: Apr 2013 Posts: 82 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2013 Posts: 82 | Thanks for the advice. I have read both manuals. Neither show where to fill the transmission. | | | | | Joined: Feb 2001 Posts: 1,094 'Bolter | | 'Bolter Joined: Feb 2001 Posts: 1,094 | I have never had a 4 sp in my truck, but I seem to remember someone saying to take the shifter out and fill from there. It is easier when trans is in the truck already.
I would think that the trans is no different than the 3sp and has a plug on the side, fill to the point where oil just reaches the bottom of the plug hole.
Tommy
59 apache 1/2t
261 short stepside
| | | | | Joined: Oct 2006 Posts: 9,671 'Bolter | | 'Bolter Joined: Oct 2006 Posts: 9,671 | Half way up the right side of the transmission you will see a pipe plug. Fill with oil till you can touch it when you poke your pinky finger thru the hole. If you're just topping off the oil a squeeze bottle such that the oil comes in works. If you've drained it and need to add the 6 pints that it holds then it is faster to pull the shifter as Tom mentioned. I've filled an empty 4speed from the fill hole on the side before but it takes a half hour to do it that way. Then again, it takes at least a half hour to pull back the floor mat, remove the transmission cover and pull the stick and put it all back together. http://www.pbase.com/dennygraham/image/137678406 Denny G.
Last edited by Denny Graham; 05/02/2013 7:55 AM.
Denny G
Sandwich, IL
| | | | | Joined: Apr 2013 Posts: 82 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2013 Posts: 82 | Thanks. Can I use 80w90 GL5 in both the rear end and transmission? | | | | | Joined: Oct 2006 Posts: 9,671 'Bolter | | 'Bolter Joined: Oct 2006 Posts: 9,671 |
Denny G
Sandwich, IL
| | | | | Joined: Apr 2013 Posts: 82 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2013 Posts: 82 | ok, drained my 4 speed transmission today. It is bolted to a 235 Chevy. Oil was thick and black. it was ell above the fill plug on the side. I pulled the side cover of since I could not remove the drain plug. Refilled through side plug with 80w90 gear oil. Want to make sure I'm not missing something. Is there any way to figure out what transmission I have? | | | | | Joined: Apr 2013 Posts: 82 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2013 Posts: 82 | In the same vain when I was underneath, it looks like there is a fill plug where the drive shave goes into the differential. Should it be filled with 80w90? | | |
|
|
