You are absolutely right. iIt is about changing the rubber spark plug. I had similar issues earlier and my mechanic friend did same..wait, youre getting blue smoke coming from under your hood? or youre getting blue smoke coming from your exhaust?
if blue smoke is coming from your exhaust, it has to be getting into the motor somehow. when you did the valvecover, did you change the 4 rubber spark plug tube gaskets on the valvecover? when was the last time you did a tuneup? normally oil can pass over old rings or thru a bad headgasket, but seeing you have a 91, you must consider other vacuum items that could be shooting oil down your intake, eventually out your exhaust. things like discount hid kits, egr, pcv, etc could also cause this.