Can Radial and inverted engines use multigrade?
Generally the safe bet is no. The main reason is that the downward pointing cylinders tend to collect oil when the engine is not in use and this can cause a phenomenon called hydraulic lock when the engine is started. When this happens, the oil partially fills the combustion space and, being incompressible, increases the pressure in the cylinder enormously when the piston moves on the compression stroke and can result in severe engine damage. It is an attempt to remove this oil that radial and inverted engine owners pull the propeller through prior to start. Obviously if an oil is more fluid at ambient temperature, then this problem is more likely to happen, so multigrades are generally not used in this type of engine. There are exceptions, such as small radials in arctic conditions, or Gypsy Major engines, which have been shown to work once they have a modified oil control ring etc., but generally these engines tend to use monograde oils. If in doubt – seek advice from your engineer,
