AFAIK the st and rs had a specific oil filter, i got a std filter when i changed the oil and afterwards i had what i would say was tappety noise before the oil got to the top end of the engine, lasted about 6 secs, changed to pukka filter and it is now 3 secs or less still annoys me as its oil not getting to the head or draining while sat up and when the starter motor cranks all to life the noise starts.. again its not hydro tappets but solid to the best of my knowledge, ran it past an Expert Ford man and he said, its a zetec.. they do that!!!