In the last couple of weeks my gearbox has become hard to change gear, it feels like the oil in the box has turned to treacle. It goes into gears fine with no grinding, so I don't think it's the clutch, but moving left to right when in neutral it is very sluggish. 

Would an oil change help, but then why would the original oil thicken up? There's no leaks that I can see underneath the car.

Any help appreciated before I take it to the garage. 

The car is a 2010 which has just covered 100000k 

Changing the gearbox oil seems like a very good first move.

That is why there are drain and fill plugs on things as they really are not filled for life, well not actually for ever and ever and ever.

?

Have you had the car from new so know the oil has been in since 2010, or could it have had a service previously and with who knows what?

Edited 12 December, 20205 yr by e-Roottoot

Lubricate the remote control cables & linkage, reset the linkage while you are at it (youtube), unlikely to be gearbox oil related.

As JR said it's the linkage connections, buy some  PTE spray and lubricate the linkage and cable ends on the gear change remote should sort this issue out

I have previously had this issue which was the mechanism on the top of the gearbox. A  spray with WD40, followed up with the oil can cured this. get someone to do the left to right gearlever action while you look under the bonnet to identify relevant/required movements.

Will check and try all of the above.

Thanks

Have a look at this link