mark79,80

Posted a recent thread about rear end noise on a 1979 Corvette.
Several replies recommended using Lucas 85w-140. Owners
manual calls for 80W-90. What is the advantage of using the
higher viscosity? Any potential problems?

martinsw

I have a 79 (w/LS1 & T56) - this summer, when hot, my diff was making noises - on heavy and slight turns - so bad I thought I might have to rebuild the diff. I read somewhere that the heavy duty Lucas 85W-140 would help w high temps, so I put it in (w/2 bottles of GM additive). Figure worth the try before dropping the pumpkin.

Noises went away , have driven about 500 miles since change - no problems I am aware of...

Anyone know of drawbacks to running 85W-140???

gkull

Thicker oils just masks the problem. Like if you have lifter noise with 10W-30 just put in 20W-50. You didn't fix anything! You can also just turn up the music to fix the problem.

Thicker oil is for higher heat and pressures. Your LS1 might be working your rear end and building heat. My 79 pumpkin gets so hot that you can't touch it for 30 minutes after shut down. So I use 85W-140 synthetic.

panic

i do use the best 90/140 fully sint oil money can buy in my diff as well, specially because it's an uprated rebuild with tom's parts and 4.11 gears (and i'm not easy on them at anytime... )

gkull

Panic, I also have a 4.11 rearend. 4.11 rear gears produce more heat than lower numbers like 3.73..........

tjhuyser

you can get 85-140 in regular gear oil...there is no issue with it because the cool viscosity is essentially the same..

the lower (85w or 80w) is the thickness (viscosity) of the oil at 0 degrees F.. depending on where you live this may or may not be a problem..if you are dealing with a High performance Vette, it isn't a problem cause you ain't drivin it in 0 degrees !! so for all intents and purposes the 85w and 80w are interchangeable, the big difference is the viscosity at the top end...it is rated at 212F, which means you are going to have lubricating properties closer to the cooler temps at 212 with the 140 than with the 90. Still with me?? ok I know the rear in a tractor trailer generally runs about 230- 260 when loaded...I don't know what the average temp of a Vette rear is going down the road, but I imagine it'll be less (lower friction, less weight, less stress) so I figure you'd be good with either, however i prefer to go with the 140, unscientifically, my thought is the polymers can expand more so there is less stress so it lubricates for longer.. Now the big difference would be in the Lucas, lucas has a lot of synthetics and has a fantastic record in the trucking industry, I highly recommend them,, the oil tends to cling to metal parts more..they have both 80-90 and 85-140, you can try the 80-90 first since it is what is recommended, but I think the fact that it's Lucas would make a bigger difference then the weight