DDK - Die Deutschen Klassiker

To add to Strictly’s comment I looked on Pelican for evidence (fact/myth/opinion….) of Swepco being the thing to use and found a couple of statements from seemingly good Porsche shops who supported race cars, saying that Swepco was the only thing to use as it protected the synchros so well. I tried it even though I don’t race and couldn’t see an improvement. It’s difficult to say whether my gearbox will last longer with Swepco as there isn’t really a baseline from which to assess it. What I do know is that when I replaced the Swepco with Motul Transynd 75/90 it shifted much more readily.

I think the consensus from most gearbox ‘guru’s’ for the 911/912 gearboxes, is that Swepco is too slippery for the older type gearboxes, and their synchro hubs.

Whilst oil technology might have moved on, the moving parts in old Porsche gearboxes has not, so just use what the Porsche original designed for them. There is a reason EP 80/90 oils are still available and have not been replaced.
I would have thought you would have been better off with a good MINERAL EP90 as opposed to synthetic, which is not ideal for old style Porsche-type synchros, and apparently does not give enough friction for the synchros. Porsche engineers in the 1960/70’s specifically factored in the gear oil to help slow down the synchro's during engagement. Synthetic oils do not help the pieces slow down and cause premature wear. Synthetic might not make the baulk-ring synchros of the 901/915 boxes work well.

If you speak with Mike Bainbridge he will give you an idea what to use. He always recommends an EP 80 or 90 MINERAL OIL

I think Mike closed his company doors last year, all his web site addresses seem to be down.

I think Mike closed his company doors last year, all his web site addresses seem to be down.
Mike rebuilt my various 915 boxes over the last 20 years and always insisted on the original spec oil, this for my road going car and race car (this box has a LSD non-plated type)

Last year I changed that type of oil to Motul HD 80/90 Mineral for the hill climb 915 box and felt little difference.

It is imho very hard to say there is a noticeable change of gear shift feeling with these oils except when I tried Swepco which was noticeably poor and drained after 3 weeks. That was years ago.

Possibly, the Motul gave better shifts when ambient cold, seems much the same when hot!

As to wear...you will never be able to say this or that oil reduces wear, but I bet the old 915 synchro dog will wear faster with Swepco.

How much oil is needed for a 901 dogleg gearbox? Is it 2.5 liters?