WELCOME to LMF....
Your title is Oil... but then you ask about changing spark plug??
So I'll guess you meant to ask about both...
If you use a good Synthetic oil you could change Once a Year, or every 100 hours, whatever comes first....
And the spark plug should be changed, when it Needs changing... I have a Cub Cadet with a spark plug that has been in it for Over 30 years... I remove and check the gap every Other year...
I have other small engines with their Original plugs, and they are all over 5 years, most over 10 years old... if the electrodes are gaped correct, and they are still working, Why would you think about changing them?? :smile:KennyV
