CRJ-200 technician reported that the main oil filter was not installed on the engine during engine build up. The Technician reported that this was discovered during routine maintenance later.

Narrative

On Aircraft X; maintenance in ZZZ hanger found an oil filter missing from L/H engine during 'A' check. Soon after I was notified that it was my signature on the previous paperwork for in shop maintenance before the engine was installed on Aircraft X. Having the paperwork for the oil filter been dated for DATE; the engine installed on DATE1; and this being found on DATE2 it is very hard for me to recall weather or not I did this very specific engine oil filter. During that time I do recall us doing a lot of engine work back to back usually having 2 engines being worked on in the shop at the same time; training in a new guy and at times hopping back and fourth from them. Looking back at PMI part orders for the engine oil filter 2 of significance came up; one being for a W/O (Work Order) on DATE3 which I typically always use while working in the engine shop because it's easier to remember that than a new/different W/O number every time; also I will typically compile a big list with parts for an engine and any other component work orders that I have been working on to save time from going to the computer every 10-20 min with new parts. I also backtracked a fuel filter kit being picked on the same day (DATE3) because I will typically do those filters together; while it had my usual 37340 w/o number the issue number was separately picked. I usually wouldn't think much about that but something to note was that during those months we we're training in a new guy on day shift and according to the w/o for the engine he was working on the engine with me so that could of defiantly played a part in it getting missed by me or forgotten by him. This guy on many occasions; especially when he first started; would say that he completed something yet later on we would find out that he didn't; I can't speak for why people would give a definite answer when they aren't sure themselves; which is why I can't give a definite answer for if I signed that I did the filter or if I signed for someone else doing it. The other one was for a W/O on DATE4 for an engine assembly that wasn't that engine. I have found out that parts room does not save the sheet for who made and signed out the issued parts order because the filter was ordered to the engine shop work order and not a specific aircraft or engine W/O. It is because of that; working and training on the engine with a new guy; the extensive amount of time it has been sitting around the hanger for anyone to touch and also having an inexperienced contract maintenance team installing the engine for either their first or second time ever; that at this time I can not confirm or deny that I was the one who forgot or misplaced the filter. But mechanics in ZZZ said it happened and my name was on the line.For part pick lists; they should probably be saving all of the pick list orders (i.e.; who made it and who signed it out) in the future so that one can identify if they picked the part. On the flip side of that I probably should of been using the w/o for that specific engine. Having specific trainers or having it more known that if your training someone you should just focus on that and not try and do other work while half training someone.