remove the top radiator pipe at the engine to check, For oil in that engine I prefer to use a thinner viscosity like 10W40 or 5W30, it moves quick to fill the lifters. However your engine has been running too cold for a long time, there would a lot of sticky oil residue in there.
Hot engine oils work to keep the engine clean as the toxic gases and water etc are steamed out and then sucked by the PCV into the intake.