Manual Trans Gear Oil
#1
I have been reading it's best to change the trans gear oil on a rebuilt manual trans at about 500 miles to remove any metal particles and assembly lube.  Then I read you should only use a gear oil with a GL4 rating and not the more current GL5 rating.  Apparently GL5 with cause excessive wear on soft metal parts like the blocker rings and thrust washers.  I have been using Lucus gear oil that states it meets both GL4 and GL5 ratings.  So any thoughts on this topic?????  It's not easy to find gear oils with only a GL4 rating.  I have found one from Joe Gibbs Racing at Summit Racing.  Everything in local in parts stores is GL5 or combined GL4 and GL5 rated.

On another note for those of us with Traction Lok differentials or any other clutch type posi rear axle.  A GL5 rated gear oil is apparently slippery enough to not need a friction modifier additive.  I can at least say the Lucus gear oil I am using works just fine in my 9" Traction Lok differential without any other additives.
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#2
Hey Mike,

I do recommend dumping fluids after 500 miles or so on any fresh rebuild whether it is a engine, transmission, or differential to flush out any break in particles from new parts.

The gear oil topic has become a confusing one in modern days and loaded with opinions and preferences.  As you mentioned, the manufacturer recommended gear lubes are typically are no longer available leaving us to find substitutes.  To make matters worse, there are many more choices to choose from and when you ask the question you will get different answers.

If my memory serves me correctly, I am using Lucas 80/90 in my transmission and Valvoline 80/90 for my differential with no problems or performance issues.  I did not add any friction modifiers to the differential because the Valvoline already contains it.   Like you mentioned, I believe the Valvoline 80/90 says it is not recommended for non-synchronized transmissions most likely due to the softer metals.

I hope this helps.

Mike
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#3
Thanks for the input.  I think I'll stick with the Lucus 80W/90 gear oil I already have.  It does state on the bottle it's for transmissions and differentials.  Next time I do an oil change maybe I'll order the Joe Gibbs Racing 80W/90 gear oil.  That one is GL4 and specifies it's for synchronized manual trans.
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#4
Update:  I called Lucus Oil this morning.  According to the person I spoke to Lucus does not have a gear oil they recommend for older synchronized manual transmissions.  Everything they offer is GL-5 rated and not recommended for older transmissions.  I guess I will be ordering the Joe Gibbs Racing GL-4 gear oil from Summit Racing.
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#5
(02-25-2019, 12:05 AM)Mustangmike Wrote: Hey Mike,

I do recommend dumping fluids after 500 miles or so on any fresh rebuild whether it is a engine, transmission, or differential to flush out any break in particles from new parts.

So how do you change the differential oil?  My 9" rear axle does not have a drain plug.
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#6
(02-26-2019, 05:33 AM)1969_Mach1 Wrote:
(02-25-2019, 12:05 AM)Mustangmike Wrote: Hey Mike,

I do recommend dumping fluids after 500 miles or so on any fresh rebuild whether it is a engine, transmission, or differential to flush out any break in particles from new parts.

So how do you change the differential oil?  My 9" rear axle does not have a drain plug.
I am lucky with my Mustang because it has a drain plug on the bottom.  It has a 57 Fairlane housing and apparently they came with a drain plug because I did not put one in it.

As far as the Fairlane, the housing does not have a drain plug so I am just going to syphon the old oil out through the fill hole and put new oil in.  I wish had put a drain plug in it when it was out of the car.
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#7
Is there enough room to syphon through the fill hole?  I hope so because that going to be my best option as well.  The ring gear and carrier are pretty close to that fill hole.
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#8
(02-26-2019, 10:15 AM)1969_Mach1 Wrote: Is there enough room to syphon through the fill hole?  I hope so because that going to be my best option as well.  The ring gear and carrier are pretty close to that fill hole.

As long as you find a narrow enough tube you should be good.  I used one of those pumps for filling the differential and the transmissions. They work great.  The only problem is 80/90 is so thick your arm gets tired pumping.  I think most part stores sell pumps and syphoning kits. I would take your car for a nice ride and pull it right into the garage and syphon the gear lube when the differential is warm. This way the particles are less likely to be lying on the bottom of the differential and it should be easier to syphon out when the gear oil is warm.  

A lot of people criticize this method because they claim it doesn't remove the metal filings.  Let me know what new car manufacturer tells you to change your differential fluid after 500 or 1,000 miles?  There are a lot of cars out there that probably never had the differential fluid changed once.
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#9
When I took a break today I decided to take my 69 Mach1 out for a short ride.  This is the first time since draining the Lucus GL4 and GL5 rated 80W-90 gear oil and refilling it with the correct 80W-90 GL4 rated only Joe Gibbs Racing gear oil.  What the heck, the trans is so much harder to shift than it was with the Lucus gear oil.  I might go back to the Lucus oil.  Is the Lucus GL4 and GL5 rated oil truly bad for these toploader trans or is that a bunch of speculation by and nobody truly knows?  Because by sitting in the seat and pulling on the shifter Lucus seams to be a better gear oil.
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#10
Truly the gear oils we have today, are so far improved, over what was originally used in our cars. I would say that if the Lucas shifted easier, I'd stay with it. As it's head, and shoulders above the regular "Hypoid" from the 70"s, as far as protection goes. I've used Lucas, in a lot of standard transmissions, and rear ends. The improvement is always immediately noticeable. Case in point my 2008 Mustang 4.0 V6 With the 5 speed. From the day it was new, it was always difficult to downshift from 3rd to 2nd. It was also quite notchy at first, in all gears, which I expected. As the car was brand new and had 7.3 miles when I sat my butt in the seat for the first time. It got slightly better, but was still hard to down shift, and super fast upshifts would sometimes bind up, and not be able to completed at all. (Missed). the car had about 60,000 miles, and recommended changing the trans fluid. Which is ATF. So I decided to add about 1/2 quart of Lucas oil stabilizer to the ATF. The difference was immediate, all shifts were improved up or down. I could now speed shift at full throttle, with no lock out. The car now has 165,000 thousand miles, and it is still perfect. I also had to replace the Bearings in the rear end, about 90,000 miles. No real reason, just started making noise. I put new bearings in, and 1/2 quart of Lucas. Now it's quiet as a mouse. I've have always used Lucas in Big Trucks, and have seen all the advantages to that. So I'm sold on Lucas. I definitely believe it protects better then most anything else.  Wink
   Just don't use it in later vehicles that require a 5w or 0w weight oil as it's to thick, and can cause oil starvation at cold start up. I liked to have blown up our school's Trash truck a 2000 GMC with a 5.3 Ls motor requiring 5w 30, and it got very tight one cold morning after I had just changed the oil, and added Lucas. Almost locked up solid. We let it set for a couple hours in the warm shop, started and ran fine after it got warm. Changed back to the 5w 30 it called for, and no Lucas. That was a year ago, still runs like a top. So don't use Lucas on later model engines, the clearances are just to tight. Hope it helps. 

JTS
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