Did you really mean a 15W-20 blend?
If so, get that oil out of there and change to a synthetic diesel rated CI4, CI4+ or CJ4 5W40 oil or a 10W30.
Also, if you really did mean it, post a link to the oil.
Yes Motorcraft Super Duty Diesel synthetic blend. Oh Shoot~ it is a 15W-40 not 15W-20! DUH! I talked to my part store guy with your concern. He ordered up Amsoil full synth diesel 10W-30 for me.