i have a 89 RS with a 305 TBI. underneath used to be bone dry, no leaks at all. now i have slight oil leak and i dont know where it is comming from. i checked all of my valve cover bolts. several of them where quite loose. i re-tightened all of them. i think it might be the rear main seal but i dont know. what is the easiest why to tell. could the valve cover gaskets be trash? the leak is not severe but it is noticeable when you work on the car from underneath. are the centerbolt valve cover gaskets prone to failure?

Take your car to a car wash that has the underside sprayer when you drive in. Then take it home and see if you can spot some oil seeping out. It's next to impossible to find a slow leak if there's oil everywhere.
A RMS leak wouldn't have any oil anywhere forward of the very back, and anywhere up above that level. It's fairly easy to spot that one.

Oil along the block? Like where the freeze plugs are? Then it's not the oil pan. Is it only on the front of the engine? Most likely the timing chain.

If it's mid height, and along the sides, you're right, valve cover gaskets most likely. Those are pretty cheap, might as well grab a new set and throw 'em on. And no, centerbolts are supposed to be more leak-free, hence the center-bolts.

i think i can rule out rear main seal there is oil on the outside of the pan. i dont think its the pan, it doesnt appear to be comming from around the gasket surface of the pan. my car is very clean so the pan is still in like-new condition. the front of the engine is dry as a bone so i can rule out timming chain cover. the valve cover are very suspicious, espcially considering how loose several of the bolts were. does the gasket get ruined when you just re tighten the bolts? will just retighen them usaully reseal them?

what valve covers do you have?
Alloy or tin (crome)?
Mine leak like a b!tc# and nothing i do seems to stop them, I need to p!$$ these crome ones off and get a nice alloy set.

If it wasent for my valve covers my block would be so clean but instead I leave a puddle of oil where ever I park.