Every small engine on a lawn mower that uses an internal combustion engine needs a constant stream of lubrication in the crankcase. This lubrication comes from engine oil, which also helps to keep the engine cool, venting heat away from the cylinder. If the engine oil is low or the wrong kind is used, the lawn mower's engine will quickly overheat and shut down.
The Best Oil
With so many different brands of automotive engine oils on the market it's hard to know which one is the best for your lawn mower's small engine. However, look for several key factors for the oil you decide on. All lawn mower engine oil should meet the API (American Petroleum Institute) service rating of SG-SL and should only come from the highest quality detergent oils. While most lawn mower engine manufacturers will vary their oil specifications slightly, one of the best for most temperatures is an oil rated at SAE 30.
In certain weather conditions, such as cold weather below 32 F (0 C), change the oil in the crankcase to an oil better suited to it. A common oil to use in colder weather is SAE 10W 30 or SAE 5W 30. These two oils are considered multi-viscosity oils because they perform like an SAE 10 or 5 oil, respectively, for cold weather starts. They will still operate like an SAE 30 as the engine heats up.
Nearly every high quality SAE 30 oil will suffice for most operating circumstances. However, multi-viscosity oils may burn oil quicker if used at temperatures above 32 F. So if you decide on a multi-viscosity oil such as a 10W 30, check the oil supply more frequently to ensure adequate lubrication exists at all times in the crankcase. Never use reclaimed or used automotive oils for lubrication in your lawn mower. Always follow your manufacturer's lubrication specifications for your lawn mower.
The small engines on lawn mowers need regular oil changes just like your automobile. Most manufacturers will recommend changing the oil in push mowers after every 25 hours of operation and 50 hours of operation for lawn tractors and riding lawn mowers. If your mower doesn't regularly reach this number of work hours, change the oil at least once a season. Clean the oil filter during every oil change and replace the filter after every other oil change.