RV-10 pilot reported observing oil on the windscreen during departure. The pilot returned to the departure airport and landed safely.

Narrative

My safety pilot and I took off ZZZ on Day 0 at approximately XA:30. I was (pilot in command) PIC and am the airplane owner. My plan was to execute 3 practice instrument approaches in VMC to maintain instrument currency/ proficiency. I had transitioned from the Tower frequency to ZZZ Departure and had just gone under the hood when my safety pilot asked about the moisture he was seeing on the windscreen. I looked up and saw oil streaming out of the oil door on the top cowling. Realizing that this could be a very bad situation; I immediately requested a frequency change back to ZZZ Tower and made the change. I asked ZZZ Tower for immediate landing and then; upon recommendation from my safety pilot; [requested priority handling]. ZZZ Tower cleared me to land and asked the nature of my [problem]. I advised oil on the windscreen. I performed an [priority] descent with power at idle and performed an uneventful landing. I taxied back to my hangar shut down and began the cleanup.The cause of the oil stream was a missing dipstick. (The engine is an IO-540-D4A5). I had interrupted my pre-flight and neglected to reinstall the dipstick after topping off with a quart of oil. I recently had changed my procedure from keeping the dipstick on the wing to hanging it from a hook on my bench (out of plain sight!) because I have a fresh paint job. The combination of interrupted pre-flight; hanging the dipstick on the bench and not having a spring on the oil door to keep it open when unlocked resulted in my error.Moving forward; I will return to keeping the dipstick on the wing near the pilot door when adding oil and installing a spring on the oil door to keep it open when not locked. These two items should alleviate a missing dipstick scenario.