The Good Oil ?

[MD]

To my disappointment I have been duped for years by the promo blurb on the so called good oil for the Alfa transaxle. Slippery this and synthetic that and the more the better. Well, let's take a closer look at the way the Porsche style synchromesh functions in these boxes. The mechanism requires some friction ability to slow down the spinning gear to mesh it. A very slippery oil interferes with this requirement. Sure, these synthetic wiz bang oils are good for the gearsets but not for the synchro function.

What is required is a traditional mineral based oil such as Castrol 85/90 Universal (or equivalent) which is made specifically for this type of synchromesh.

It's all a trade off. What is important to you? Synchro saving and function or the gearset? Given the situation for new replacement parts for these transaxles being almost unobtainium, it requires some individual ponder.

[Colin Edwards]

No doubt that different lubricants and their particular attributes affect synchro performance.
Although I do give the transaxle on the 75 some time to "warm up", what's important to me is relatively smooth and easy gear shifting without puttering around for 15 minutes waiting for the transaxle oil to thin down a bit.

Presently running Castrol Syntrax Universal Plus 75W/90 in the 75.  It's claimed to be fully synthetic with a viscosity index of around 160. Being a tad on the thin side, synchro performance is fair when cold and pretty good after say 5 minutes. 

Most Alfa specialists seem to recommend a specific oil for the transaxle.  As long as its semi synthetic or fully synthetic, has good shear stability, has friction modifier if equipped with an LSD AND supports acceptable synchro performance................?
 

[MD]

Good to see another opinion Colin. The site has been lacking contributions of late..be that as it may.

I think I need to add a tad more to my initial post for clarity of my view. Fully synthetic oils for the Alfa Transaxle will work and will work as well as expected while the synchro sets are new. What the problem is that they are working too hard accelerating the wear because the oil does not have sufficient  friction for it to work as designed. It is why the mineral based oil is recommended.

Gentle and shifting by feel when the box is cold is a given. It will never shift like an MX5 doesn't matter what you put in it. However, it will do a good job for a long time if the correct oils are used.

[Colin Edwards]

Funny you mention the MX5.  I have a 124 Abarth Spider.  The gearbox in this is essentially out of an MX5 NC!  When cold, the gearchange was terrible - far worse than the transaxle on my 75!!  Apparently, this is a common complaint re this box.  Supposedly fixed in the MX5 ND.  I tried a number of different lubricants in the 124 box.  Nulon 75W/85 made a reasonable improvement, however as the 124 is almost my daily drive, very cold mornings were still an issue till the box warmed and the oil thinned a bit.  Final fix was Penrite Pro Gear 70W/75.  As I recall this oil has a very high viscosity index so it won't thin much when hot.  Important for a relatively thin oil - don't want it thinning much further!

Granted the "slipperiness" of an oil has an impact on synchro performance, however methinks viscosity also plays a major role.  I prefer a full synthetic as these generally have a higher viscosity index (less affected by heat) and usually (but not always) a higher film strength.  I'm not sure synthetic oil = a more "slippery" oil or a mineral oil is less "slippery" than a full synthetic.

[MD]

I am no oil chemist so I will just simply say, any regular Joe running an Alfa transxale would do well to read the Castrol Product Data Sheet on the Castrol 80W90 Universal transmission fluid referenced here:

https://msdspds.castrol.com/bpglis/FusionPDS.nsf/Files/9F7435227CDB408980257E2D0081371E/$File/bpxe-9vy7we.pdf

[GTVeloce]

I confess to knowing little regarding oils however, I was put onto Redline Lightweight Shockproof for the transaxle and have been very happy with it. I don't notice any cold weather issues (even in the middle of a Melbourne winter) and one of my cars lives outdoors. I also haven't had any real significant wear in the snchros or gearset over the past 10 years I have been running it. I also haven't changed it since I put it in... 

https://www.redlineoil.com/lightweight-shockproof