Oregon residents are legally permitted to apply tinting material to their side and rear windows provided that the film has:
A light reflectance of 13% or greater
A light transmittance of 50% or more
The capacity to create a total light transmittance through the window of 35% or greater
If you have a medical condition that could benefit from tinting, you can also get a letter from your physician or optometrist permitting the use of tinting material with a total light transmittance of under 35%.
If you want to tint a rear portion of a multi-purpose vehicle, there are no restrictions around light transmittance; you have much more freedom to decide on the tint that's best for you rather than the tint that best abides by the rules.