myself I have mixed 95% C3 oil with 5% HDEO (Delvac MX15W40) to bring up the additive levels a notch.
No side effects during that OCI.
Think of it like this: You go on a trip, have top stop for gas and then you see your oil level is low. But the choice is limited at the rural gas station. Mixing whatever they have is better than risking a oil level too low.