75W-90 or 75W-140 in my Dana 60?

[70Sbird]

I just had my 68 Dana 60 bearings replaced and am putting it back into the car this week. The shop that did the bearing replacement recommended 100% Synthetic gear lube, but they didn't have a clear answer on the weight other than to refer to my service manual. I've been doing some Googling and opinions are all over the board, including some pretty heated debates on some truck forums. The 69 Dodge FSM calls for the 80W-90 wt oil with the Sure-Grip additive. I know that was the correct answer 50 years ago, but is there any advantage to going to a 75W-140 over a 75W-90. This is a street car I use for long stretches on the highway and local driving as well in summer months, no racing, towing etc...
Thanks for looking!
Scott

[c00nhunterjoe]

I run 75/90 in all 3 of mine. 2 are street driven and see 130 mph at the track, the 3rd is track only and sees 155 mph. No failures on any of them.

[cdr]

When I bought my new Strange S60, they DO NOT recommend synthetic,, they send 85/140 Lucas with their rear ends when you buy one.

[c00nhunterjoe]

When I bought my new Strange S60, they DO NOT recommend synthetic,, they send 85/140 Lucas with their rear ends when you buy one.

Did you get a trak lok or the strac diff?

[cdr]

When I bought my new Strange S60, they DO NOT recommend synthetic,, they send 85/140 Lucas with their rear ends when you buy one.

Did you get a trak lok or the strac diff?

The S trac

[c00nhunterjoe]

I beleive the helical locking diffs do not react well to full synthetics. We did not run it in the military versions either.

[John_Kunkel]

Yeah, from their website"

"Gear Lube DO NOT USE SYNTHETIC GEAR OIL Will adversely affect unit operation and void warranty."

[70Sbird]

Interesting, but if they are using a "Sure Grip" system that does not play nice with Synthetics I understand. But I'm still curious about my 50 year old unit. I believe I have run synthetic lube in it before with no adverse effects, I'm just trying to do what's best for the axle since I don't want to ever see it out again.....

[cdr]

Interesting, but if they are using a "Sure Grip" system that does not play nice with Synthetics I understand. But I'm still curious about my 50 year old unit. I believe I have run synthetic lube in it before with no adverse effects, I'm just trying to do what's best for the axle since I don't want to ever see it out again.....

If you had no problem running Syn gear oil, then why did you rplace the bearings ?

[Birdflu]

When servicing a Dana 60 with either the 1st generation Powr-Lok or the later used Trac-Lok units, we solely use 80W-90 GL-5 standard petroleum based gear lube in our shop. The clutch type units generally work better with petroleum based lubricants.

[70Sbird]

Interesting, but if they are using a "Sure Grip" system that does not play nice with Synthetics I understand. But I'm still curious about my 50 year old unit. I believe I have run synthetic lube in it before with no adverse effects, I'm just trying to do what's best for the axle since I don't want to ever see it out again.....

If you had no problem running Syn gear oil, then why did you rplace the bearings ?

CDR, I replaced the bearings as more of a maintenance or "peace of mind" item. They have been there for 50 years and I have some long (2,000 miles plus) road trips coming up in the next year or two starting this October. I pulled the rear axle to fix a persistent pinon seal leak and to replace the springs. In the last 20 years that I have owned the car, I have changed out the rear gear lube a couple of times, once after I bought it, and then again a few years later to replace the cover gasket. I'll be honest, I don't remember what all I have used but I'm pretty sure the last fill was Mobil 1. The guy who replaced the bearings said they looked good for their age, and he didn't see anything they was near failure, but at least now I know everything is good to go with the axle assembly. I have no problems running Dino oil, synthetic, of any combination of weights. I was just curious if any of the options are better than others. 

[John_Kunkel]

Since the introduction of synthetics, I have never used anything else in Sure-Grips; but the S-Trac is different.

[c00nhunterjoe]

Since the introduction of synthetics, I have never used anything else in Sure-Grips; but the S-Trac is different.

Agreed. Torsion diffs are the exception.