A lawnmower engine runs at speeds much higher than an automobile, and most often, they run in dirtier and tougher conditions than most automobile engines. For this reason, lawnmower engines need a constant supply of lubrication, from oil that meets their viscosity requirements. Always check with your manufacturer's oil specifications for any special requirements.
Why Oil's Important
The oil inside your lawnmower engine works exactly like the oil in your car or truck. First, the oil lubricates the piston to keep it from grinding on the walls of the cylinder. Second, the oil transfers internal engine heat from the engine to the cylinder block and out the exhaust port. This allows the lawnmower engine to continue burning fuel at a high enough temperature to maintain velocity. Without this lubrication, the engine would quickly heat up from the increase in friction, and the seals around the piston and crankcase would get damaged.
Engine Oil Ratings
All petroleum-based engine oils are given a rating system, or SAE weight, to determine their viscosities and the ideal temperature that the oil performs best at. Most engine oils nowadays use a multi-viscosity oil, which allows them to perform better at a wider range of temperatures and viscosities. Oil manufacturers add a Viscosity Index improver to the base oil mixture. So, oils with a rating of SAE 5W-30 will perform at lower temperatures like an SAE 5W oil and lubricate at higher outside temperatures like an SAE 30W oil.
The Recommended Oil
Most lawnmower manufacturers recommend an oil with an SAE 30 rating. This gives the engine proper lubrication in the widest range of temperatures. However, with ambient temperatures below 4.44 degrees Celsius, a thinner, less viscous oil may work better, like an oil rated 5W-SAE 30. When operating in temperatures consistently above 4.44 degrees Celsius, a 10W-SAE 30 will work best under most circumstances. While operating conditions are important to consider, remember to always check with your manufacturer's guidelines for any special oil types.
Checking Oil Regularly
Lawnmower engines tend to burn through oil pretty quickly, and generally, most lawnmower companies recommend changing the oil every 25 to 30 hours of operation. For this reason, check the oil level regularly, ideally before and after every use. This will allow you to monitor the performance of your engine oil and make any adjustments as necessary. If the oil level drops too quickly, move to a more viscous oil; if the engine oil level never drops, switch to a thinner-weighted oil.