get rid of the K&N. https://youtu.be/gNQ6d3ox5Ok
try and go back to stock if you can....
as for why the oil has lower vis. is because, these years "regen" they dump fuel on the exhaust stroke to burn build up outa the DPF, in turn it thins the oil.
VPB is a fine oil.
you should look into "deleting" the emission stuff. it'll let your truck live a longer life.