Sears Lawnmower problem

I have a Sears Craftsman lawnmower that won't start anymore. I'm not sure how old it is (it was a hand-me-down from my father) but it's at least several years old. Had to get a new blade last year. Within the past 3 years I've drained (by turning it on it's side) and replaced the oil a couple of times. I use 10W-30 or 10W-40 car oil. After I replaced the oil a few weeks ago, the mower started to have more difficulty running. It sounded and felt like it was struggling with less and less power. Finally it got to the point where it was very weak. Now it will not start . (It is self-propelled and has a pull-cord to start it.) Is this related to oil? Could some oil have leaked into a wrong place? Is there something else that should be opened for cleaning/inspection? How can I tell if the spark plug is too old and needs to be replaced? How long does a lawnmower usually last?
Thank you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
replace plug, your carberator may be a problem, how long do they last depends on maintenance and hrs run, load , etc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.