Place a drip pan or other container on the side of the mower with the filler. Carefully tilt the mower so oil drains out and into the pan. Discard the spent oil into a proper disposal vessel. Now slowly pour fresh oil into the mower. For the specific type of oil you need check your instruction manual. Make sure to add only as much as your particular model requires. Remember to let the mower sit undisturbed for a few minutes so the oil settles properly inside the engine.
Your Best Digs also recommends the Sun Joe mower but the reviewer also added a number of caveats. He found it will get the job done if you prefer to only bag and just have $100 to spend. However the clippings compartment filled up quickly for him. And the height adjuster felt flimsy and required adjustments on each wheel.
The EGO LM2101 is the highest-rated electric-battery mower on Consumer Reports by far. Its lowest category rating is for its bagging for which it is rated “good.” Despite testing the push option the reviewer found this mower was easy to operate and had effortless handling. However they found the 40-minute battery runtime to be fairly short a common issue with cordless electric mowers. Techaeris tested the EGO LM2102SP and found the battery lasted 60 minutes which was long enough to finish the reviewers 1/3-acre lawn. He estimated that the yearly cost to operate the EGO would be $1.50.