Oil buildup on spark plug

[tino0217]

About a year ago on my 71 C10 I know the truck was missing when I drive when I check the spark plugs on the driver side first plug towards the front there was oil deposit and build up which was making the truck miss. I had a leak in my oil pan I had somebody come out and change it and the valve covers now in the last week the truck was missing again so I checked the plugs thinking it was that same one but it wasn't I checked the passenger side plugs and it was a third one back that had oil buildup on it which is making the truck Miss anybody have any ideas why this happened