Turn on one side only, pressing on both sides can make tighter. I learned this from fixing camera lenses with thin threaded rings without the correct spanner tools. A sharp catch on one side and rotate it out, put a clean rag in the hole to catch any little chips.
A small sharp screwdriver or maybe just your finger is enough to turn it out. The threads are not tight on these. I've never seen a tight oil cap thread, just the gasket gets tight..