Pulled the plugs on my impreza 50,000 miles and saw traces of oil, not sure
what this means, friend said could be valve cover gaskets needs replacing, what
could this mean?

P356 wrote:

It could be your friend meant "valve guide seals." Valve cover gaskets
cause external leaks onto the ground. Valve guide seals are internal,
and help slow the flow of oil down the valve guides into the combustion
chamber. Although 50k MIGHT be a bit soon for them to go, if you drive
in a hot climate or keep the engine wound pretty tight, it would not be
unheard of. How's the oil consumption? Bad seals often cause a lot of
oil to be burned with little or no apparent smoke.

Best of luck with this!

Rick