80w-90 or 85w-140 for heavy highway use

[SS1970chrysler]

Car is 1978 Ford Granada 6cyl engine, C4 auto, and WER axle w/ 2.75 ratio. Still on original fill w/ 41k miles. Used for 90 miles highway daily.
Would I benefit from the heavier oil or would it throw my mileage into the toilet? Very little to no aftermarket support for this axle and I'd like it to last.
That, and I still have a couple unopened qts of 85w-140 left over from another project.

 

[qdeezie]

If it was me, I'd go for synthetic 75w-90 gear oil.

 

[kasedian]

Originally Posted By: qdeezie
If it was me, I'd go for synthetic 75w-90 gear oil.


+1

 

[Boczech]

Originally Posted By: qdeezie
If it was me, I'd go for synthetic 75w-90 gear oil.


+1

Nothing wrong with a good synthetic gear oil. Dang I must be getting old. When I was taking drivers education in high school we had an 1978 Ford Granada. WOW that was some crazy times back then....

 

[oilboy123]

I run 75w-90 as a rule, if your towing maybe heavier weight. I remember drivers ed and our vehicle was a Chevy Nova that was about a 76 year. That dates me........

 

[SS1970chrysler]

I took my driver's test in a 76 Chev Impala and bought a 77 Granada shortly after High School. I'm moving up in the world.

No towing or excess cargo beyond spare tire, bag of tools, my lunch, and 10 pounds of extra parts. Car stays at roughly 60mph and hits 65mph for short spurts.

Any benefit to even lighter weight than what I have in stock. I've always been told that you go one grade heavier for extended highway use. If I've been misled all these years, enlighten me. I'm still willing to learn sometimes.

 

[oilboy123]

If you look at a good synthetic 75w-90 gear lube at room temp it almost looks like the thickness of water. So synthetic in that wt flows better in cold temps than heavier dino or heavier wt synthetic.

I see no reason to go heavier than a 75w-90 wt dino or syn unless you tow. I can see maybe one weight heavier if towing, but if you use a good synthetic I wouldn't bother.

Off the Amsoil site for a 1980 Granada:
They show their 75w-110 as the heaviest wt.

Standard Differential, Rear W/ Rear Disc BRK......GL-5
All TEMPS......90
SEVERE GEAR 75W-90 Synthetic EP Lubricant (SVGQT)
75W-90 Synthetic Long Life Gear Lube (FGRQT)
SEVERE GEAR 75W-110 Synthetic EP Lubricant (SVTQT)
SAE 80W-90 Synthetic Gear Lube (AGLQT)
Standard Differential, Rear......GL-5
All TEMPS......90
SEVERE GEAR 75W-90 Synthetic EP Lubricant (SVGQT)
75W-90 Synthetic Long Life Gear Lube (FGRQT)
SEVERE GEAR 75W-110 Synthetic EP Lubricant (SVTQT)
SAE 80W-90 Synthetic Gear Lube (AGLQT)
Limited Slip Differential, Rear W/ Rear Disc BRK......GL-5*
All TEMPS......90 [4]
SEVERE GEAR 75W-90 Synthetic EP Lubricant (SVGQT)
75W-90 Synthetic Long Life Gear Lube (FGRQT)
SEVERE GEAR 75W-110 Synthetic EP Lubricant (SVTQT)
SAE 80W-90 Synthetic Gear Lube (AGLQT)
Standard Differential, Rear W/ Cover removed......GL-5
All TEMPS......90
SEVERE GEAR 75W-90 Synthetic EP Lubricant (SVGQT)
75W-90 Synthetic Long Life Gear Lube (FGRQT)
SEVERE GEAR 75W-110 Synthetic EP Lubricant (SVTQT)
SAE 80W-90 Synthetic Gear Lube (AGLQT)
Limited Slip Differential, Rear W/ Cover removed......GL-5*
All TEMPS......90 [4]
SEVERE GEAR 75W-90 Synthetic EP Lubricant (SVGQT)
75W-90 Synthetic Long Life Gear Lube (FGRQT)
SEVERE GEAR 75W-110 Synthetic EP Lubricant (SVTQT)
SAE 80W-90 Synthetic Gear Lube (AGLQT)
Limited Slip Differential, Rear......GL-5*
All TEMPS......90 [4]
SEVERE GEAR 75W-90 Synthetic EP Lubricant (SVGQT)
75W-90 Synthetic Long Life Gear Lube (FGRQT)
SEVERE GEAR 75W-110 Synthetic EP Lubricant (SVTQT)
SAE 80W-90 Synthetic Gear Lube (AGLQT)