[Hib Halverson]

90s don't have valve stem seals on the exhaust, consequently, 90s tend to use oil and start to smoke much sooner in age than do 91-95.

While that blue smoke thing could be injectors–and you can determine if it is by running the fuel pressure tests in the FSM–it's possible that it may be time to pull the heads and have the intake valve seals replaced and have them installed on the exhausts.