I know motor oil blocks some of the suns rays. I know this because when I get it on my photogrey glasses, the oil spot stays clear and the rest will darken. I dont know which spectrum it is blocking but some of it is not getting thru the oil. This is not a recommendation to go oiling down your tractor as it will for sure attract a large amount of dust.
Me, I will hose mine off to remove the majority of dust and dirt and hose all the mud off when I finish a task and put it up for storage, until then, it get hosed only when the operator platform starts getting me dirty.
I think you can safely pressure wash a tractor if you dont direct the nozzle directly at a gasketed joint at a very close distance. The gasket should hold and not be damaged if you keep your wand at least a couple feet away from these areas. I rountinely pressure was my engines but I dont stick the nozzle 2 inches from the engine surface. I have never had an issue with any of my cars, trucks or tractors being damaged by pressure washing to remove the built up dirt and grime. The engines run a lot cooler when clean also.