[Schulisco]

If you do not know what kind of gear oil is filled, I recommend to think about changing the gesr oil. The 915 uses the Porsche patended synchronization which requires more drag torque to function the synchros properly. Modern oils like part or full synthetic gear oils does not have that much of drag torque, although Porsche still recommends even modern (part) synthetic oils to be used in the 915. Originally Porsche recommended and filled a simple mineral only based SAE90 GL5 gear oil. Mostly Shell Spirax MB90. On shifting problems like yours I would use such an oil for a test. This is by far cheaper than an overhaul and might cure your problem. If not you know the synchros might worn out...compare the datasheet of the MB90 to others choose a similar equipped oil by yourself. The viscosity values are the important thing...

Thomas