Background...
DP and I have lived together a few years, no kids. We're not on vastly different incomes so we split joint expenses such as rent. Everything else we pay for individually, including transport costs. I own a car which I drive to work as my workplace can't be reached by public transport. DP doesn't own a car as he buses to work.
The issue...
DP has a hobby he does occasionally at weekends which requires travelling, often to out-of-the way places, think gravel roads, towns several hours away, muddy areas. DP borrows my car for this but it almost always comes back a bit dirty (e.g. mud splashes on the outside, or bits of gravel, sand etc inside. Memorably it once came back with a lot of dead sandflies inside). So not in a terrible or irreversible state, but not up to the standard I normally keep it.
I get a bit annoyed when this happens as I generally keep my car clean, it's also work funded vehicle so should be kept presentable. I've also been quite vocal that if he wants to borrow it then although it doesn't have to come back spotless, it should be the same condition it was in when he borrowed it. He's repeatedly returned it a bit untidy, although claims he's made an effort to check it over before it comes back.
I'm also not happy with him driving it to towns 1-2 hours away, as it's my only means of transport to work and I'm responsible for its maintenance. It's a small, old car (not a 4 wheel drive or anything like that) so I worry about the wear and tear on it going to some of these places. I also worry (maybe irrationally) about the impact on me getting to work if anything should happen to it. I'm sure he'd happily contribute more towards it financially, but I'd almost rather he didn't as I'd then feel obliged to let him drive it whenever.
He wants to borrow the car next weekend to travel to a town 1.5 hours away. AIBU to say no? Or am I just being precious and selfish? I've no issue with him taking the car within our city to see friends, go to appointments, shopping etc, I'm just uncomfortable with the hobby trips for the reasons above.