Some radial installations had an oil dilution switch to reduce cold weather starting torque and a boil-off procedure to restore oil viscosity after start.
Oil dilution doesn't "reduce starting torque," but reduces oil viscosity by thinning with avgas. This allows for easier oil flow in the absence of preheating, during the initial start. Most dilution systems are deactivated or removed. Preheating and preoiling is always a better choice.
How do you preheat a Merlin, or for that matter, any water cooled aircraft engine? Is there such a thing as a block heater for these engines?
The same way one preheats any aircraft engine.