any reason why i shouldn't use 10w 40 semi synthetic in my x type with 168 thousand on the clock.

figured it could use a slightly thicker lube as its bound to be a bit worn!!!?

As a long term owner of all types of cars I've found that most multigrade oils (excluding EP gear oils) will be fine. Alternatively there's little if any financial advantage in not using the recommended 5W 30 fully synthetic. However I snaffled up 14 gallons of the stuff from my local ASDA late last year when they were flogging a lot of it off at half price.

*Smugface*

If, as you surmise, the engine is showing signs of wear and is using oil then it's reasonable to go for something a bit thicker. Or would be if 10W 40 was thicker when warm; which it isn't. I can hear the howls of anguish as I type this, but if you have to go thicker, then go thicker. By that I mean 20W 50. But only if you're seeing a significant loss from one change to the next. If not, stick to 5W 30 and maybe chuck in a bottle of Slick 50. I've been using it for decades with no detectable harm in many Jags and other makes.