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.
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.
Other items and supplies include a plastic drip tray motor oil and a container for waste oil. Often a large piece of cardboard is useful as well an old rag or cloth. Also know that this guide deals with push or self-propelled mowers not riding mowers. While some advice here applies they are another animal altogether.