How often should i put wd 40 in my truck?

Uhm... This thread is a little vague... Where do you intend to put WD-40?

I use WD for my bearings after I clean them and I also use WD to clean my cars and trucks with. IT makes the parts look like new. I normally spray it on a paint brush and then put in on the car and then when done I wipe the car off with a rag and it looks like new.

Never put wd40 in the engine use After run oil for that. As for cleaning it with wd40 thats fine although some people say it dries out the plastic bits.