How to Change Lawn Mower Oil: No Fancy Tools Needed!

© Eugene Brennan

Motor oil is an essential requirement of all engines. It has several functions, including lubrication of metal parts in contact (reducing friction), cooling, keeping surfaces clean, sealing and corrosion inhibition.
Your 4-stroke, walk-behind lawn mower engine is no different. The oil inside it gets dirty over time and without an oil filter to remove metal particles and soot, these contaminants can build up over time and cause engine wear. So it’s good policy as part of a maintenance schedule to give it an oil change after every 20 to 50 hours of use (consult the manual or check the engine block for exact details).

What Time of Year Should I Change the Oil in My Mower?

Ideally change oil at the end of the cutting season, before the mower is stored away for the winter. Combustion products in oil such as tars, carbon, sulfur and nitrogen dioxides can settle out, coating engine parts or possibly causing corrosion, so it’s best to remove it and replace it with fresh oil.

Safety First While Working on a Lawn Mower

• Don’t run a lawn mower in an enclosed space. Carbon monoxide gas, which is present in exhaust fumes, is odourless, colourless and tasteless, but it’s poisonous and can kill. Ideally, run a mower outdoors when testing. If the mower is run in a garage/workshop, it should only be operated for a short period and you must ensure all doors and windows are open in order to provide adequate ventilation.
• Set controls to off and remove the spark plug lead before working on any moving parts on an engine e.g. the blade. Ideally, you should also remove the spark plug to be totally sure that the engine won’t fire if you turn the blade.
• Keep petrol (gas) away from any sources of ignition.

What Tools Are Required to Change Oil?

• Socket wrench set or a wrench(spanner) to fit the drain plug
• Spark plug wrench
• Shallow container for collecting drained oil. A biscuit (cookie) tin is ideal
• Some cloths for mopping spillage and wiping hands
• Disposable gloves
• Some newspapers/plastic sheeting for covering the floor or ground surface
• Bricks or short pieces of 2″ timber for raising the mower off the ground

A shallow tin like this one is ideal for draining oil into. Metal tins aren’t totally leak proof and oil will eventually ooze out, so dispose of it safely
© Eugene Brennan

What Type of Oil Is Used in a Lawn Mower?

Generally, an SAE 30 or 10W30 multigrade oil is recommended. Sometimes 15W40, which is more viscous when hot is recommended for larger engines. Check your manual for the required quantity and type of oil.

Checking oil level

During the cutting season, keep the oil level under the max level on the dipstick. To check oil:

1. Move your mower to a level surface.
2. Remove the dipstick.
3. Clean oil off the end of the dipstick with paper towel or a rag.
4. Replace the dipstick. Some engine manufacturers recommend that you don’t screw it back in again.
5. Remove the dipstick again and check where the level of the oil is between the max and min marks on the stick.

Step 1: Find a Location

Pick a suitable location for working on, ideally the concrete floor of your garage, or you can work outdoors on a driveway.

Step 2: Keep Your Working Area Clean!

To avoid mess, cover the working surface with a few newspapers, plastic sheets or refuse sacks (trash bags) to collect any drips/spills.

You can use a refuse sack (trash bag) for covering the ground or floor.
© Eugene Brennan

Cover the floor with a refuse sack (trash bag), newspapers, plastic sheet etc.
© Eugene Brennan

Step 3: Add A Little Fuel to the Lawn Mower Tank

Pour a small amount of petrol into the tank, enough to allow it to run, so that it doesn’t leak out when you turn the mower on its side. Alternatively, drain petrol after running the engine.

Step 4: Run the Engine to Warm the Oil

Run the engine for about 5 minutes to warm up the oil. This makes it less viscous and more fluid and it will drain easier. Don’t run it any longer than this or the oil will scald you if it gets on your hands while draining!
Turn off the fuel tap if fitted after running the engine.

Step 5: Remove the Spark Lead

Caution !!!

Detach the spark plug lead and ideally remove the spark plug from the engine to prevent it from starting.

Step 6: Decide Whether to Drain Through the Plug or the Oil Fill

The oil in a 4-stroke engine is held in a reservoir at the lowest point called the sump. Usually, an oil drain plug is screwed into the underside of the sump and when this is removed, oil can be easily drained. Any sludge which has settled should drain also.

What if the engine doesn’t have a drain plug?

Not all engines have a drain plug and oil must be drained from the oil fill plug by tilting the mower.

Step 7: Tilt the Mower to Access the Drain Plug

If you decide to empty the oil through the drain plug, you need to either turn the mower on its side or onto its back handles to access it or at least tilt it so that you can get at the drain plug with a wrench. Check with the instructions or the manufacturer/your dealer first to see which method is recommended. If you turn the mower on its side, turn off the fuel tap if fitted. The float bowl on some engines is vented and fuel can trickle out this vent and into the intake manifold or onto the engine. If this happens when the engine is hot, it can potentially start a fire, so ideally drain fuel from the bowl first via the drain plug.

WARNING !!!
Remember, remove the spark lead and ideally the spark plug also to prevent the engine from starting if you have to move the blade to access the oil drain plug.

Some manufacturers recommend placing the mower on its side. The breather is usually on the same side as the carburetor, so keeping it upright prevents oil from leaking out and fouling the carb or air cleaner
© Eugene Brennan

Alternatively lower the mower back onto its handles. A weight such as a block will keep it in place.
© Eugene Brennan

The breather, a vent to the oil containing sump on the underside of the engine, is usually on the side of the engine adjacent to the air filter and carburetor. Keep it upright to avoid an oil leak
© Eugene Brennan

Step 8: Locate the Drain Plug

Locate the drain plug. This may look like the hex head of a bolt in which case you can loosen it with a socket wrench. If you don’t have a socket set, you can use an open-ended wrench (spanner). Alternatively, the plug may have a square head and you will need to use an open-ended wrench. Some plugs are recessed and flush with the sump with a hex socket or square socket head, requiring an Allen key or the 1/2 inch drive of a socket wrench respectively to be loosened. Turn the blade of the mower to move it out of the way if it is obstructing access. You can also remove it. In general, the bolt which fastens a lawn mower blade in place is loosened by turning it counter-clockwise.

Drain plug on a mower.
© Eugene Brennan

Oil drain plug.
© Eugene Brennan

Step 9: Loosen the Drain Plug

Loosen the plug, but don’t remove it fully. Turn the mower back onto its wheels and position it over the tin. If you raise the cutting height to the highest setting, it may be easier to accommodate the tin under the deck of the mower. You can also raise the front and back wheels onto short slabs of timber or bricks to increase clearance so that you can get your hands under the mower. Just be careful that it doesn’t roll off these and drop down onto your hands while removing the drain plug.

Step 10: Drain the Oil Into a Tin

Now this is the difficult bit. You need to fully unscrew and remove the plug and move your hand quickly to one side out of the way. Disposable gloves will keep your hands clean but sometimes it is difficult to work with these. If you work bare handed, ensure you wash your hands well with a cleanser to remove oil and use moisturizer. Allow the mower to drain for several minutes into the tin and then tip it slightly from side to side to remove any remaining oil.

If you remove the fill plug from the top of the engine, oil will drain quicker because the sump is vented. However, don’t forget to replace it before tipping the mower back on its side to replace the drain plug. Otherwise some oil may trickle out the fill hole.

You can raise the wheels up on pieces of timber or bricks for added clearance to fit the tin underneath. Roll up your sleeves and wear disposable gloves while draining.
© Eugene Brennan

I drained oil into this tin…..
© Eugene Brennan

WARNING !!!
Don’t overtighten the drain plug. If it’s plastic you can split it in two, or worse still, if it’s metal you can cause thread damage to the sump.

……..and bent a “spout” with a pliers to make it easier to empty the tin.
© Eugene Brennan

Step 11: Replace the Drain Plug

Turn the mower back on its side and replace the drain plug. Don’t over-tighten it, especially if it is made from plastic. Wipe away any oil from the underside of the deck.

Step 12: Add the Recommended Type and Amount of Oil to the Engine

Add the new oil to the engine through the fill plug. A funnel helps to avoid mess. Small bottles of oil specially for mowers sometimes have a narrow spout, so a funnel isn’t necessary.

How much oil is needed for a lawn mower engine?

For small to medium-sized mowers, you need from 17 to 20 fluid ounces (U.S.) or 500 to 600 ml of oil. Usually, there are graduations on the side of small bottles of oil so that you can ascertain how much you have poured out. However, I find it more convenient to use a plastic milk container which I have placed a mark on the side of, indicating the required amount of oil to fill my mower. I keep this container from year to year and I fill up to the mark from the oil can, and then just empty this amount of oil into the mower.

Oil level should be between the high and low mark on the dipstick
© Eugene Brennan

How Often Does Oil Need to be Changed?

Generally every 25 to 50 hours or every season, whichever comes first. Check your engine manual for the recommended interval between changes.
Sometimes it’s recommended that oil in a new engine is changed after 5 hours, so again, check with your dealer/the engine manual.

Related Reading

How to Fix a Lawn Mower That Won’t Start

References

Castrol United States (English). (n.d.). Oil viscosity chart: Motor oil grades: High Performance Engine Oil, Fluids & Lubricants. United States (English). Retrieved April 1, 2023, from https://www.castrol.com/en_gb/united-kingdom/home/car-engine-oil-and-fluids/engine-oils/engine-oil-viscosity-grades.html

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2013 Eugene Brennan

Further Reading