Oil + additive viscosity

Don't use Lucas Oil Stabiliser. It dilutes the additives in your engine oil because it has very little of its own and can cause air entrainment. If you need more viscosity, use a heavier engine instead. See Lucas Oil Stabilizer.

Engines need oil flow and pressure is a characteristic of flow and viscosity. Excessively thick oil will just be relieved back to the sump without getting to the bearings. See Engine Wear.

What is the reason that you think you need a higher viscosity?