Looked all over the web for answers, nothing specific and most answers were clutch & gearbox oil viscosity related.
My issue was just a stiff gears at the gearstick in all directions, engine off, engine on.
There are two gear cables, one for fwd & back, one for left & right. Inside the engine bay they are protected by metallic heat shields as they are routed over the top of the exhaust manifold.
Changing them looked a complete pita on YouTube so I figured that they were maybe needing lubrication.
Remove plastic battery vent and plastics above airbox.
Loosen intake clamp to Inlet manifold.
Unclip airbox top, remove air filter and entire top with Inlet pipe still attached.
Give lower airbox a big tug and it comes out.
Then you can reach under the battery and feel the linkages.
They just pop off with a chisel type ball joint splitter or a large screwdriver as a lever. Be careful though.
Then you can push the cables back & forth to feel for stiffness. Mine were very stiff.
I used high temp ceramic brake grease, diluted with some naphtha to make it runny. Then used a syringe to drip drops onto the cable inner as I exercised it back & forth. Took about 10 minutes, drip and exercise, to get the lube up the cables.

Fill the rubber boots with lube before locating them back over the linkage recess. This extra lube will make its way along the cables by capillary action over time.
Gearchange was immediately better & has improved further as the grease migrates further along the cables every drive.
The possible reason? Maybe my cables had become dry and stiff from over 150k miles sat above a hot exhaust manifold.
So, much cheaper and easier than replacing the cables for anyone else with this issue.