Engine Runs Slightly Rough

A couple of days ago coming to a stop light, I started noticing the Engine  
performing slightly rough or vibrating slightly off it's normal operation feel.
(Best I can describe what I am feeling from the engine). I owned the car
since 2005, had a few issues come up that couldn't be handled with a look at
forums, or repair sites with those issues, this one has got me stumped. Only
thing I had done recently is change the Oil using Royal Purple, everything
seemed fine, but this came up shortly after, not sure if it's related but I am
assured by many online that RP is a really good synthetic. The car itself
Starts up fine, runs fine, doesn't have any power loss, it's just the way the
engine is vibrating when started, feels alittle rough, but only alittle. Since I
have driven this car for so long I know how it's normal operation feels,
something here is just feels slightly off, not sure what it is. I am leaning
toward the oil I used but maybe that's not the problem. . .