Might be the piston pins that you hear. Any way what I would do is to change the oil right NOW! If it ain't cold where you are I'd go with straight 50w and not even worry about changing filters, and i'd keep the rpm's as low as I could.. I really don't see how you could have hurt the engine if you did not bend the valves. Those old 258 engines have great bottoms.
Big Jim <img src="/forums/images/graemlins/wink.gif" alt="" />