LSD vs 915 synchros

[RivieraBob]

I've just had my 915 box rebuilt and 'whilst in there' had a Kaaz reverse (biased to deceleration) 1.5 LSD fitted.

Seems to me I'm now left with a compromise on oil, the plate LSD liking slippy stuff, the new synchros wanting more resistance. The LSD comes with it's ridiculously expensive own branded oil which is mineral GL5 80w90 so looks like the right spec but having spent mucho Porsche tax on new synchros I obviously want to protect them. Looking through old posts I see Mike Bainbridge has recommended to others in the past Castrol b373 for TBDs and he obviously knows his onions, but on paper it looks like the same problem.

We're all bored with oil posts and it can become a 'my wife is prettier than yours' type discussion, but it's a genuine question of how to choose an oil for two different functions. An LSD is hardly new tech in an old 915 box so there must be an experienced solution to the compromise?

Bob

[proporsche]

i will only use mineral 80W90 oil..simple as that... BOB

 

Ivan

[RivieraBob]

Sure Ivan, the Kaaz oil is mineral and to that spec.....BUT.....it's the LSD additives I worry about wrt the synchros.

[proporsche]

back in the day there use to be no additives and everything was fine;-)..what is it WRT synchro??

[RivieraBob]

ah sorry....nothing technical....wrt - with respect to

[topcarrera]

I use this with my factory ZF 40% LSD 

Castrol Axle Z Limited Slip 90

[Nick93]

I spent some time wrestling with this dilemma after a rebuild of my SC's 915 'box with factory lsd. There are synthetic transmission oils which will work in a lsd transaxle but apparently 915 type synchros do not work well with synthetic oils.  There are GL5 spec mineral oils but I did not find one which would meet the requirements of both the lsd diff and a synchromesh gearbox. In fact the spec sheets of quite a number of lsd compatible oils stipulate that they are not suitable for use in synchromesh gearboxes.  This website neatly sums up the issues;

https://blueskymotorsports.com/index.php?option=com_content&view=article&id=16&Itemid=6

For what it's worth I eventually went with this stuff;

https://www.ravenol-direct.uk/ravenol-epx-80w-90-gear-oil-gl-5.html

It certainly improved the shift quality over the synthetic that was previously in there and I have not had a problem with "diff chatter" although maybe my diff is no longer as tight as it should be.

I did come across a Ferrrari 308 forum debating this topic which suggested the way forward was to fill with a suitable spec gear oil and, if diff chatter is experienced, to incrementally add friction modifier until it is no longer evident.

In the end it's a compromise.

[RivieraBob]

Thanks, that link is an excellent article summarizing the compromise....friction modifiers needed for the LSD but sub optimal for the Balk Ring 915 synchros. Was hoping for some magic juice that worked for both. Guess you have to prioritise, so new synchro protection trumps new LSD.