[Schulisco]

Follow the specs Porsche has given for the gearbox in the older days. On a 915 they always recommended a mineral only (neither part nor full synthetic!!) gear oil with SAE90 GL5!!! specification (and no GL4!). Mineral oils do have a drag torque which is necessary to perform the Porsche patended synchronization mechanism in the 915 gearboxes. Synthetic oils doesn't have this and this causes worse shifting after being warmed up.
Today Porsche recommends a 75W90 for almost everything, this makes storageing much easier for them, but my experience told me to go back to the old fashioned gear oils they recommended first. In the very last hint on the last page of the PDF they're saing that you can still use the old fashioned SAE 90 mineral oils on the 915 etc. gear boxes...



...with manual transmission types 901/902/905/911/914/915/925/930... the previous SAE90 GL5 (MIL-2105 B) approved for these transmissions can optionally continue to be used.

My experience with the SAE90 mineral oil: Far better shifting when warmed up, a bit worse when cold, yes, but hey - I don't use the car to do daily business.

Thomas