I have had lots of problems with leaking speedo and late super rocket style oil pump drive tachometer cables.
What I discovered was that the oil was leaking out from between the sheath and the aluminum fitting that is crimped over the end of the sheath so no amount of tightening and adding sealant or replacing O-rings will solve the problem. My cure was simply to carefully cut through the crimp on the aluminum fitting so I could slide it off of the cable sheath. I then removed all of the old fabric type covering on the outside of the original sheath. Finally I cut a piece of rubber tubing the length of the sheath and slid the entire sheath in the length of tubing. I then replaced the crimped fitting which now fits over the old sheath and inside of the rubber tubing and put a hose clamp on each end of the rubber tubing. Oil leak eliminated. I have also had new replacement cables where the oil leaks out between the crimped on aluminum fitting and the new plastic coated cable sheath.
Lastly, the BSA A65 tachometer cable rotates in the opposite direction of an A10 oil pump drive tach. I believe the A65 cable will fit on the A1O but the sheath and or the cable may be wound in the opposite direction. If you put the A65 cable on an A10 the cable and or sheath could essentially use screw action to pull oil out of the motor and fill your tachometer with oil. I personally have not had this experience but I talked to a fellow A10 rider whose tachometer kept filling with oil and the cable being wound in the wrong direction was the only cause we could think of that made any sense. A theory to debate I suppose.