the insides are a bit convoluted so you cant do a mint cleaning job but you can bring it back from being brown or orange.

the last one i cleaned i used cat litter and washing powder to sort of scour the insides. boiling water helps-stick the cap on and block the return hole. shake vigorously. obviously throughly flush out. sticking a jet wash inside might jet out some crap.

i wonder how well those mini valeting steamers would clean one up?

failing that new ones from Fiat are about £30 and finish off a clean engine bay nicely.