well i figured was running a little low on oil about a quart so i got the synthetic figuring start with blend then due full oil change, now i have a clunking noise on start up and at idle on certain occasions. There is no miss, hesitation or low oil pressure at all. At times i think might be the cat convertor since it never has been changed. BTW engine smooth and at high speed doesnt do it only certain times any ideas? Not engine ping since that would be at all times...go back conventional? Also developed no leaks from putting little synth in..

 

It wont be the oil that is giving you the noise synthetic oil will mix with conventional oil no problem. sounds like something else to me