What type of printer are you using?
Material-wise, I use IJ680CR with 8518 laminate. The Oracal is too dark for Engineer Grade, and Arlon requires I bake the s.o.b. for it to stick to a vehicle.
Printing: for the Mutoh printers, I modified a 3M Prem G profile to make it work for me and I doublestrike it so the colors kick. The HP at the 9-5'er, I use the IJ180 profile in Onyx, but tell the printer that it's IJ680CR, so it prints 12p but heats up properly for the IJ680CR.