Should I Add Oil Each Time I Use My Lawn Mower (Four-Cycle Engine)?

The Proper Way to Check and Change Your Oil

Closeup of lawn mower cutting grass.
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If you are conscientious about mower maintenance (but don't know much about four-cycle engines), you might wonder if you should add oil each time you use your lawn mower. Is this a good idea? Or is it misguided? If you are prone to carrying good intentions too far and overdoing things, you will want to read on.

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

The proper procedure is as follows for four-cycle engines (also called "four-stroke" engines):

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.

To check the oil, begin by making 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, I do this twice (double-check). 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:

  1. If the oil appears clean but the reading is low, add oil. 
  2. 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), drain out the lawn mower's oil -- it will come out easily, since the engine is still rather warm at this point. Once the old oil has drained out, replace it with new oil.
  1. If you have both dirty oil and a low reading, add a little oil before mowing, then follow the steps laid out in the prior paragraph. Sure, this way, you'll 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're mowing.

    Filling the Engine With Oil: the 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:

    What can go wrong when you under-fill the engine:

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

    What can go wrong when you over-fill the engine:

    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.

    In light of these facts, it is necessary to exercise caution when filling your lawn mower engine with oil. It helps to 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).

    What About 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 not any oil to "check." The only checking comes in making sure that you are using the recommended type of oil, and using it in the recommended ratio.