I think it used be in the Vauxhall hand book, something about a cleaned car drove better, and lasted longer.
Mini's got washed when dirty, it was the easiest way of checking for rust on the inside-out seams.
I once had a new Mini treated with Waxoyl preservative. (not sure it is still available but a treatment involved drilling holes in all the hollow section and then inserting a mini spray nozzle and coating the inside)
The car was coated everywhere with a soft wax carried in a solvent, the idea was it would penetrate into all the nooks and crannies. (my spelling checker insists it should be grannies!)
Anyway, when I collected the car, the guy said, there's good news and bad news, good news is it's finished.
Oh god, big breath, what's the bad?
Don't wash it for a month!
OK, breath out...