What type of differental oil is required in 1955 Thunderbird

I’m performing some springtime maintenance on my 1955 Thunderbird before the start of cruising season and since I have never changed the rear differential oil I was wondering what kind or brand of oil should I use?

Referring to the service manual, it says that the rear differential requires 3½ pints of SAE 90 multipurpose gear lubricant. Since there is nothing wrong with the differential on my T-Bird, I don’t want to cause any problems by using the wrong type of oil.
doug7740
1955 Thunderbird Blue
 
I’m performing some springtime maintenance on my 1955 Thunderbird before the start of cruising season and since I have never changed the rear differential oil I was wondering what kind or brand of oil should I use?

Referring to the service manual, it says that the rear differential requires 3½ pints of SAE 90 multipurpose gear lubricant. Since there is nothing wrong with the differential on my T-Bird, I don’t want to cause any problems by using the wrong type of oil.
doug7740
1955 Thunderbird Blue
Great question Doug, I look forward to any responses. I have to change my rear end seal in the next few weeks and have been assuming regular gear oil shouldn't be an issue.
 
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CSPIDY

Well-Known Member
I don't usually trash talk products but Royal Purple is not a very good product from my personal experience. for the rear ends in our early birds a non synthetic 85-145 is best. this is advise was given to me by the drive line shop that worked my rear end this past winter. (this is all they work on)
unless you are racing you car or using it as a daily driver, full synthetics add no value.
 
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Dale Leonard

Blown Valkyrie Rider
Never had a problem with Royal Purple or synthetic in older differentials. Only had a problem if there was an existing problem that I did not know about, such as a worn bearing or seal ect:
 
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After reading everyone’s comments I became more confused on what kind of differential oil to use in my 1955 Thunderbird, so I asked Gil Baumgartner who is the CTCI Authenticity Chairman. Gil suggested that I use NAPN SAE 90W mineral gear oil, because it is as originally used in early Thunderbird transmissions and differentials.

doug7740
1955 Thunderbird Blue

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CSPIDY

Well-Known Member
After reading everyone’s comments I became more confused on what kind of differential oil to use in my 1955 Thunderbird, so I asked Gil Baumgartner who is the CTCI Authenticity Chairman. Gil suggested that I use NAPN SAE 90W mineral gear oil, because it is as originally used in early Thunderbird transmissions and differentials.

doug7740
1955 Thunderbird Blue

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Full synthetics will not harm your differential, I was told the only issue may be if you let your car sit for long periods of time the mineral base ooils will leave a beaver coating on the parts above the level of the oil and less likely to rust. I apologize if I added to the confusion.
 
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biddle

Forum Moderator
Staff member
I was concerned that the synthetic lubricant would damage the old school bearings.
The only concern I've ever heard about regarding synthetic oils is with motor oils. Old cars have sludge buildup in seals that actually are preventing leaks. Newer synthetic oils have detergents that could possibly reveal these leaks. How could a product that has less heat and friction and is more consistent than real oil cause damage? Synthetic oils are far superior.

Here is a good blog on synthetic motor oils for classic cars- https://www.bellperformance.com/blog/synthetic-oils-in-classic-cars

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64ZCODE

Well-Known Member
Agreed CSPIDY. I continue to hear from some that converting a motor to synthetic oil will result in blue smoke out the tailpipe and oil leaking on the ground, which isn't the case with today's synthetics. But the old wives won't seem to let go of it:)

I think synthetic would be fine for the differential and would reduce friction and wear. Where I understand you shouldn't use a synthetic is in an automatic transmission, which depends on a certain amount of friction to function properly.

One other factoid while I'm revved up on this subject: a few years back, Mustangs and 5.0 Magazine published the results of dyno tests for a Fox 5.0 motor which was converted to synthetic oil, and demonstrated an average of 5 HP increase at the rear wheels after the conversion. Clear evidence of less friction in the motor and more whoopee for the driver.

Doug
 
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