I don't know much about other manufacturers, but Volvo uses a system of solenoids and phasing pulleys on the cams to change cam timing. There is also a position sensor on each cam so the ECM always knows where the cam is and can adjust to compensate. In this system, viscosity has no effect as the system will just adjust the flow through the solenoids to compensate.
However, the solenoids have been known to get plugged with garbage from hard carbon in improperly maintained engines. That's about the extent of our CVVT issues.