Which oil to LSD differential

[SamiA]

Experts,

Could you please again educate me. This time about the oil that I should use with LSD differential. The differential is said to be LSD from CSL. I do not know the gear nor the slip ratio. What I do know, is that the ratio is not the original for my 1973 CSi as the speedometer shows some 15% too much.

I did try to find info about the oil type from this great forum, but unfortunately did not find a clear answer.

And why do I need the oil type. Well while storing the car for cold winter months I drove the car from cold outside to warm garage. While preparing the hypernation I noticed oil on the floor on the right side of the differential. So I'm preparing to get the differential seals changed. I did order the seals and o-ring from W&N. The o-ring I ordered as the differential is wet around the left side of the diff even if the actual leak in on the righthand side. Or at least the oil on the floor was on that side.

It also bothers me why is the differential seal leaking. I'm assuming it has something to do the temperature increase. But even that should not cause the leak. I mean there is a breather on top the differential which should allow the oil to warm up without leak, right? So I'm assuming the breather is somehow clocked. I tried to order the breather from W&N, but it was not available.

With great gratitude, SamiA

 

[Christoph]

The differential is said to be LSD from CSL. I do not know the gear nor the slip ratio. What I do know, is that the ratio is not the original for my 1973 CSi as the speedometer shows some 15% too much.

E3 & E9 came with three different gear sets. The ratio is stamped on the lower right side of the housing.
3.25 - 3.0 CSi/CSL - stamped 12x39
3.64 - 2500/2.8 L/2.5 CS - stamped 11x40
3.45 - all others - stamped 11x38
Don't know about the carbureted 3.0 CSL, however.

All E3 & E9 had open gears as standard and LSD 25% as option. Two exceptions:
2800 series 1 - standard 40%, sometimes 25%
3.0 CSL - standard 25% (as far as I know, CSL are always difficult)
An "S" and the ratio are stamped on the right side of the housing. Often you'll also find a white painted "S". Open gears are not marked.

I did try to find info about the oil type from this great forum, but unfortunately did not find a clear answer.

It seems any SAE90 hypoid oil will do. BMW gave a long list when the cars were new. Most of these oils are not available anymore, small wonder. They did not recommend anything special for the LSD. This was discussed some time ago on the German E3 forum. Two conclusions:
People use standard SAE90 oil with good result.
The limiting gear will give in sooner or later, regardless of the oil.

 

[bavbob]

My choice has always been Redline 75W/90 GL-5... go to their website and enter your car's stats. I will say that my front seal did leak AFTER filling with this so I just changed the seal and all is right.

 

[Krzysztof]

...

It also bothers me why is the differential seal leaking. I'm assuming it has something to do the temperature increase. But even that should not cause the leak. I mean there is a breather on top the differential which should allow the oil to warm up without leak, right? So I'm assuming the breather is somehow clocked. I tried to order the breather from W&N, but it was not available.

With great gratitude, SamiA

I'm far from being any "expert"... Below some thoughts in topic.

Differential oil for the LSD is different than for "open gear" due to different thermal durability of both (due to friction on the plates LSD has more demanding requirements).

Purists are adding special additive to LSD differential, helping plates to survive the heat impact once in operation (friction).

Leaks from the seals are common for older cars.

The easiest explanation is the time.
Over time oil inside is getting more aggressive (acid-like).
Time is not helping rubber to keep seal dry
Old seals not always were equipped with special lips for rotating shafts to keep the oil out.
Rust on the output shafts makes the surface brittle and oil can much easier wind out of the differential. If the rust or step made on the shaft surface is the reason, exchanging seals with not help. The output flange has to be machined (with specific seal selected later) or exchanged.

Depending on the overall condition you can start with oil change and renew the "simmering" seals but I would not feel surprised if the leak will appear.

Quite often, if the differential is quiet and not leaking a lot people decide to keep it as is as any general refurbish can be very expensive and requires very professional shop to make it properly.

Very rarely the reason of the leaking differential is clogged upper vent. Each differential and gearbox has it to unify the pressure between the outside and the mechanism as during operation pressure appearing inside (gases and oil expansion).

If the oil was not exchanged over many years, exchanging only the oil can worsen the situation as it will flush old dirt and make the diff. even more leaky.

 

[SamiA]

Thank you all!

 

[Ohmess]

I too use Redline 75W/90. As to the breather, it is held in with four 10mm bolts. It should be easy to remove and inspect and if necessary, clean.