When I pulled into the spot today and set my cool down timer, I got to thinking about our automatic transmissions and the fact that they don't circulate in park. I decided to leave the trans in neutral with the parking brake set to get a little cooling for the fluid while idling.

 

If your transmission has ever been rebuilt,the valve body may have modified so your truck will circulate fluid in park. Most transmission shops will modify the valve body and install a shift kit during a rebuild and usually that modification does indeed let the fluid circulate in park unlike it does in stock form. Just thought I would throw that out there.