"it all depends on product temperature and since it is field painted or a climate that you have little or no control over ...your climate conditions come into play, that would determine if penetrol is beneficial...usually oils become thinner and flow better but evaporate quicker when the product is heated , like setting the gallon out in sunlight to warm it up... obviously your protecting the paint from getting too cold while you store it for use but if it is cool and your conditions are cool penetrol would be a better thinning agent than mineral spirits"..
I'd say I'm painting at an average of 75-80 degrees in the shade, as my garage is not temp controlled. I'm guessing that would mean it is warm enough such that using penetrol would not be necessary? I wasn't sure how hot you were talking, but wanted to check.
Also, I am guessing that the thin nap mohair roller is the way to go? I found it odd that it seemed the rolled strokes were less noticeable with the high density foam, but again I am guessing that it because 1. It would always be less noticeable on a second coat of primer, and 2. My issue with leaving the primer in the tray too long was creating the gunk type feel in a couple of spots. Would this sound about right?
Thank you very much for your advice, it's amazing how different everyone feels about the best way to do anything related to painting. I'm sure there are multiple ways to do things right, but hundreds more possible combinations of ways to do things incredibly wrong!