Clean the head oil drainback holes as suggested, and look for sludge accumulation while the valve covers are off. If you find sludge, that's a good indication that the rings may be coked badly. Clean oil and frequent changes can help, along with a bit of moderately loaded driving once the engine is warm.
An older low mileage engine can also suffer from valve stem seal failure and guide oil leakage. This will contribute to oil burning.
I would avoid using the higher viscosity oil for now. Use a good quality mineral oil while the initial cleanup is underway. Eventually, you should be using 10W30 synthetic to help prevent sludge altogether, but it may be a waste for now. Once any heavy sludge is removed, switch to syn and good filters to help clean out remaining sludge.