RV generator engines are built with one of two oil systems. The smaller ones generally have splash systems and the larger engines will have pressurized systems.
The splash system works by whipping the oil with some sort of an agitator and splashing it all over the inside of the engine. Sooner or later all parts of the engine will get lubricated. Most agitators (dipper rods) are built onto the connecting rod as it is the one part of the engine that moves around the most. As the con rod rotates the agitator dips into the oil and throws it around. Splash doesn’t work well with vertical overhead valve engines as it is difficult to get the upper end of the engine lubricated with any degree of reliability. Most of the side valve (flathead) one cylinder generator engines will have the splash system and it has worked well in them for generations. Some of the horizontal overhead valve engines such as the ONAN KV and KY series are working well with a splash system due to careful design and having the crankcase ventilation system built into the valve cover area.
The pressurized system utilizes some type of gear pump to force oil into all critical bearing surfaces of an engine. Just about all of the vertical overhead valve engines are using pressurized systems. The pump usually has a screen on its intake side to stop large particles of dirt, metallic shavings and miscellaneous loose engine parts from getting into the pump. The more expensive engines then pass the oil through a filter and then through drilled oil passages to the bearings in the engine. As an example, the older ONANs came with the oil filter as an option. Without the filter, the oil change interval was 50 hours. With the filter, the oil change interval was 150 hours. A pressurized system can allow an engine to develop more horsepower for a given cubic inch displacement due to the positive oil input to the bearing surfaces and their ability to be loaded to a higher degree with safety.
The oil that goes into RV generator engines has changed in the last 20 years. Until the multi-viscosity oil proved themselves reliable, the major manufacturers were careful about recommending them. 30 SAE heavy duty, high detergent oil is still the old standby for these engines. As these engines (air cooled) run hotter than a liquid cooled engine, the manufacturers have gone to the heavier grades of multi-viscosity oil. 15-40 diesel engine oil is the factory shipped oil for some manufacturers. They don’t know where the unit is going and this oil is good anywhere in the country. It’s also better for the engine as the additive package in the oil is made for heavy use, dirty conditions, and high temperatures. Synthetics have been around since the advent of turbine (jet) engines. The oil has come into wider use in recent years due to its ability to sustain high temperature conditions. Whether a synthetic is good for your particular unit depends on the type of operation you normally put it to. Running the genset at the rest stop for lunch 5 or 6 times a year is not a reason to use synthetic. A good high detergent 30 SAE will suffice. If you are boondocking for a week or so, and want the air conditioner, then consider synthetic. If your motor coach has its generator running whenever you’re in it, on the road or not, then synthetic is the oil for you. One thing to think about – The purpose of oil is to cool, seal, lubricate and CLEAN the internal parts of an engine. DO NOT use non-detergent oil in your genset.
You may now start the heated discussion as to which oil is better.
Generator Jim