What Motor Oil to Use in My Lawn Mower?

By Donna Dolinar, Wyandotte County Extension Master Gardener since 2006

You are getting ready to run the lawn mower and your check the oil level. If it is low, you need to add some. The question is what kind of oil should you add: standard motor oil like you use in your car or an oil designed for small engines; or is there a different type of oil to use in a riding mower than in a walk-behind (self propelled or push).

There is one sure way to know and that is to read the owner’s manual. Don’t guess!

If you don’t have the owner’s manual, head to your computer and find it online. You will need to know the type of mower and its model number. You can also search by the engine manufacturer. If by some chance you can’t find the information online, find the phone number of the manufacturer and speak with someone voice to voice.

When you have that owner’s manual in hand, don’t discard it. Put the document in a resealable plastic bag and store it near the mower for easy reference. There is other important information besides what kind of oil to use.

It is not always a good idea to use regular motor oil in a mower.

The type of oil required will vary by the size of the engine. A small engine that is air cooled can probably operate on a small engine oil, which has different additives than an automotive oil like 10W30. Larger engines (in either a rider mower or a larger push mower) sometimes come in four-cycle. This type of engine can be quite sensitive when it comes to the types of additives in the oil you use. Sometimes SAE-30 is safe but sometimes it is not.

Some engines can only tolerate a particular grade or type of oil and others are more accommodating.

WILL THE WRONG OIL DAMAGE THE MOWER?

Maybe.

It depends on the amount of oil you added, type of mower, engine requirements, size, and overall capacity. Other factors include the climate (like extreme heat). Older mowers have different requirements.

Using an incorrect oil can void the warranty on your mower.

For most people, buying a mower, especially a rider, is a significant investment and it is foolish to risk impairing its ability to function at its best, or to start piling up repair costs unnecessarily.

HOW OFTEN SHOULD I CHECK THE OIL?

It is best to check the oil level every time before you begin to mow. You check the level by reading the dipstick, just like in a car. If you have moved the mower, let it sit for a minute or so because the oil may have sloshed around as you moved the machine. You pull the stick out; wipe it off; reinsert the dipstick; pull it out again and read the level based on the little marks. Add as necessary. Do not overfill. It really only takes a few minutes and is significantly safer than running the engine dry.

WHAT IF I USED THE WRONG OIL?

If you realize you used the wrong type or grade of oil, drain all the oil out; refill it; repeat. By removing the contaminated oil and then filling it again, you should remove the vast majority of the wrong oil and be okay. See below about how to drain the oil.

WHEN SHOULD I CHANGE THE OIL COMPLETELY?

It depends on how much you use the mower. Again, check the owner’s manual. It usually specifies how many hours the engine is run before it needs service. Obviously significant acreage will differ from someone with a small, urban lawn. A good rule of thumb is at the start of the growing season (so you start fresh) or a minimum of once per year.

  • Have all the supplies and materials on hand before you start.
  • Disconnect the sparkplug wire before you start.
  • Remove the oil plug and drain the oil into an approved (safe) container.
  • Replace the plug but don’t over tighten it.
  • Start to fill the oil tank.
  • Check the level with the dipstick periodically to avoid overfilling.
  • When the dipstick registers that the oil is filled, replace the dipstick and tighten to firm closure.
  • Reattach the sparkplug wire.
  • If the engine has an oil filter, this is a good time to check it for possible replacement.

Dispose of the used oil responsibly, preferably by taking it to a location that accepts used oil for recycling.