Changing oil on a '87 Honda FourTrax 125?

My grandfather has had this since it came out, runs like new no issues. One day i asked him the oil cap looks like it hasn't be touched in years he says "not once have i changed the oil" so my question is what would happen if we did change the oil? We're doing the good old "if it aint broke don't touch it" pic of the fourtrax is below (note our's doesn't have the cargo bin found this pic on google)Changing oil on a 87 Honda FourTrax 125

In this case the good old saying if it aint broke don't fix it doesn't apply lol change the oil before it starts turning into a thick slimy goo that eventually won't be able to get picked up or get around the engine which will then cause it to seize up if you did change it it will last longer and may just preform alot better than what it is just now

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