I had this idea and had the oil changed in my wife's Outlander thinking it was likely to help or at least do no harm. But what bad idea it was. The Mitsubishi main dealer used an oil that I believe (and they deny) was not the right spec and the shift stiffened significantly. I argued it out with them, they drove it and agreed the gear change was very stiff but said they hadn't driven it before and how do they know it wasnt always like that! I gave up in the end, looked up the oil spec myself online, ordered fresh oil and changed it myself and hey presto the shift was lovely and smooth. I told them about this and they told me becuase I'd used non-Mitsubishi oil (I couldnt find anywhere to buy their own Diaqueen oil but I used a reputable branded oil of the same weight/spec) I'd likely have gearbox trouble later on. I thanked the service manager for his time in a way which intimated I thought he was an imbecile.

It's hard to say the shift has improved over the original oil as we suffered the consequences of the wrong oil for a few weeks so the improvement when I got the right oil in was dramatic. But I think you'll get mixed views on the mertis of changing oil in a manual transmission - some say can an oil really be sealed for life in a box and not degrade and benefit from a change? Other will say that a gearbox will go 200k on the same oil and leave well alone. I regret the hassle my decision caused, but we were unlucky.

Have yet to find a gearbox that didn't benefit from fresh oil, manual or auto.

Diy the cost is so low it has to be done.

Gearbox oil change is listed in Hyundai 6th year/72K service, I thought the gear change was noticeably slicker after it was done on the i30.

Main dealer service was only £200, which struck me as reasonable, my trusted indy would have charged at least £150, wouldn't have changed gear oil and wouldn't have washed and valeted it either.