Crazy cal- you are recommending outboard oil to quiet a diesel??? Can you explain that?
Supra- you say it's a shuttle- does it have upolstered or hard seats? Sound will 'reflect' off all the hard surfaces. Curtains, upholstery, and carpet will all help deaden the sound. Fwiw, I wouldn't recommend a permanent carpet because of moisture on a steel floor, but until you convert and add something to your echo chamber, a runner or two might help.