When and How to Change the Oil in Your Lawn Mower

Proper Way to Check and Change Oil in a Four-Cycle Engine

Closeup of lawn mower cutting grass.
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If you are conscientious about your lawn mower's maintenance, then you know you must check its oil level. But it is also important to know exactly when and how to add oil or change its oil. Most mowers have four-cycle engines, and there is a proper procedure to follow to ensure that the "lifeblood" of the engine is up to par. If you are prone to carrying good intentions too far, learn why overdoing it in this task is almost as bad as forgetting to do it altogether.

How to Check, Change, and Add Oil to a Lawn Mower Engine

It is best to check the level of the oil in the engine before you start the lawn mower. If you forget to do so and have to perform the check after you have already begun running the machine, make sure you let the mower cool off before touching it. This is a common-sense yard safety policy, but many people burn themselves because they forget just how hot the machine can become after running just a short time.

When checking the oil, make sure your machine is on level ground. Then extract the dipstick, wipe it with a clean rag, insert it back in, and check its reading. Just to be on the safe side, do this a second time to confirm the reading. When you inspect the dipstick, you are checking not only for the level of the oil but also how clean or dirty it is. Your inspection could yield any of three possible results, each of which dictates a distinct course of action:

  • If the oil appears clean but the reading is low, add oil. 
  • If the your lawn mower's oil appears dirty, however, you will have to take a different tack. If the reading is not low, go ahead and do your mowing. When done mowing (and after waiting several minutes to allow some cooling to occur), change the oil. Begin by draining out the old oil. It will come out ​easily, since the engine is still rather warm at this point. Replace the old oil with new oil.
  • If you have both dirty oil and a low reading, add a little oil before mowing, then change the oil after mowing. True, you will be draining out a bit of new oil along with the old, but wasting a little oil is far preferable to taking any chance that your lawn mower will run out of oil while you are mowing.

Filling the Engine With Oil: Importance of Getting the Level Right

You have to be very careful when filling an engine with oil, whether in the process of changing the oil altogether or simply adding oil. Unfortunately, getting the level wrong (whether it be too much or too little) can have seriously bad consequences.

A few things can go wrong when you under-fill the engine:

  • Without sufficient oil to lubricate them, parts of your machine can start to break down.
  • The engine can overheat, causing damage.
  • The engine can seize up.

But over-filling the engine can also cause problems. The excess oil has to go somewhere. It will inevitably end up seeping into parts where it does not belong, potentially causing them to malfunction.

When filling your lawn mower engine with oil, pour in just a little bit at a time, then re-check the level. You are more likely to make a mistake if you are rushing through this maintenance task. Use only a type of oil suitable for your lawn mower (check the manual when in doubt).

Lubrication for Two-Cycle Engines

Lawn mower maintenance for two-cycle engines is quite another matter altogether. Lubrication for two-cycle engines (also called "two-stroke" engines) is achieved by mixing the recommended oil into the fuel. Thus, with two-cycle engines, there is no oil to check with a dipstick, because the fuel and the oil are housed together. The only checking comes in making sure that you are using the recommended type of oil and using it in the recommended ratio.

Keeping an Eye on Your Oil With an App

As in just about every other area of life, you can expect computer technology to come to our aid more and more over time in the area of mower maintenance. For some high-end mowers, such as the Cub Cadet RZT-L, there is already a Bluetooth app you can download onto your phone to remind you when it is time to change the oil (and perform other tasks related to tuning up your mower and keeping it in good working order, such as changing the air filter).