Employee changing oil on 9201 eliminator filter right side of truck. Used engine compartment boarding step to go up/down many times. Felt nagging left shoulder pain. Did't use platform or stand available. He stated not new injury. Recur aggravation of old injury/illness. Didn't treat until 2/29/08. Rcvd info. he treated on 3/3/08 Dr. diagnosis - bursitis.