There has to be ONE answer for this... let's give it a try.

The Eaton (aka automatic locking differential) units in 900 series cars and some others, need GL-5 oil. 80w90. Simple.

The question is, do they need actual limited slip additive (friction modifier)?

Eaton says yes; Volvo doesn't say anything about it. To muddy the waters further, the 4x4 message boards I look at usually say "yes" to the Eaton Locker for friction modifier, but they say "no" if you are using Redline or Mobil or Amsoil (one of the ones that has the stuff already added, apparently). Some of these guys are REALLY knowledgeable about product, especially GM with that particular unit--almost the same as Volvo has.

The Eaton unit apparently does have some sort of clutches, although not to the extent of a LSD. The Eatons lock up differently and perhaps don't need the degree of added slip to prevent harsh engagement and perhaps extend life expectancy.

So... which is it? Anyone have the answer? I've never used the friction modifier in my own cars... but that doesn't mean it shouldn't be there. In low torque/horsepower applications like the B230F or B230FT (compared to V8 power), it might not really make any difference. Chatter is a little evident from some of these units, but not so much so that you'd think to add the modifier.

I suppose knowing how they were shipped from the factory would shed light on that... if anyone happens to know.

Of course, ALL input is appreciated...
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1992 940 wagon, very low miles
as well as others.

Hi Chris, I looked into the design of that diff last year and didn't see anything that looked like it would require limited slip additive, but they have probably started recomending it in dino oil for a good reason, I'll bet it quiets down the engagement a bit.Locker rear ends are known for chattering .

It is the same basic diff that comes as an option on the GM trucks now ( by the looks of things , I think GM bought an exclusive on the product, because I couldn't find anywhere that would sell the things anymore for the Volvo axel( except volvo of course$$$$$$)

---Robert

Chris we use Amsoil 2000 75W90 in all three of our cars. 92 & 94 940's and a 97' 960. The formulation includes the additive(s) for limited slip diffs. It works like a charm, no problems to date.
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92-944, 94-944, 97-960 - Amsoil lubes throughout,, Silverstar headlights, Green Diamond Tires

Thank you. I didn't phrase it exactly the way I wanted to, but I wanted answers about 75w90 in the Eaton diff, specifically because I've heard 80w90 for the clutch operation to be right. Which is probably somewhat true, but again that might only manifest itself at the very top of the torque spectrum, such as in built 4x4 models. Not what most Volvos are pushing, even turbos and inline six cars. I run Shell Spirax 80w90 in one 900, Valvoline 80w90 in another. Or was that Pennzoil? The takeup of the ALD is pretty noticeable, but not like the clunky clutch chatter of some LSD models without friction modifiers.

I'll probably end up switching over to synthetic fluid at some point in the near future.