Redline 75w-140 or 80w-140 ?? What to use

[SXE10]

Time to change my diff oil again. I put Castrol SAF-XA in there when I first got the car as I got if free through work. It's an Australian spec'd oil that we use to use in the RWD stuff like Falcon & Transit. Now I can't remember if I used the older 75w-140 or new formulation 80w-140. The older spec had a KV @ 100°C of 25 & the new spec 29.25. Looking at Redline their 75w-140 is 27.4 & 80w-140, 26 so not much difference but the VI vary hugely between the two RL. I'm leaning towards the 75w-140 at this stage but you appreciate BITOG thoughts.
The diff is a factory fitted Torsen LSD & the OE spec is GL5 LS90/80w-90 but I find the 140 helps it hook up better & it's what is recommended for track use by TRD, not that it ever sees track time.

 

[JustinC25]

At the dealer, we use to use Conventional 80w90 GL-5 in the
Lexus IS300's of that vintage. Come to think of it, that's what every rwd Toyota product got when I worked there.
Granted I'm in the states, but I'm sure it would work fine in NZ.
If you want to use a 140, I'd go with the 80w140.

 

[Eddie]

From my recollection the 75wt has usually a better base oil and a synthetic where the 80wt is a conventional base oil. I'd pick the 75wt. Ed

 

[SXE10]

That would explain why the 75 has a VI of 191 & the 80 only 144.

 

[azsynthetic]

Originally Posted By: Eddie
From my recollection the 75wt has usually a better base oil and a synthetic where the 80wt is a conventional base oil. I'd pick the 75wt. Ed


Red Line 80W-140 is not conventional base oil. It is a fully synthetic ester base stock.

http://www.redlineoil.com/product.aspx?pid=37&pcid=4

 

[SXE10]

Ok so they're both created from polyol ester base stocks. Why the big difference in VI?

 

[edhackett]

A wider viscosity range has a higher VI by definition. You can't compare across viscosity ranges.

Ed