How oil frugal you need to be with oil in an event depends on how long you normally play each day, what your average fleet composition is, and whether you allow overruns on the canteen. If you don't play enough normally to burn your oil, or otherwise use it for dorm food, then you can amass large oil stockpiles by doing missions. Personally, on a non-event basis I'm not super frugal with oil, and I make sure my canteen never overflows. As such I never have much above my soft cap (11k currently), and when a new event rolls around I tend to buckle down on oil use.
That said, I have a month's worth of subscription oil in the mail, and a few thousand left to be redeemed from the last event, so I'm not particularly starved. It's just that by being more oil efficient I can farm more per day without worry, and it makes clearing out the entire event shop easier.
On a somewhat related note, I did the <8 fielded star condition for the Black-SP stage today, so now that's 3-starred. Not that there's any reward for doing so, but I figured I'd do it just in case it ends up being one of the ??? pocket diary conditions. Fielded Unicorn, Hood, QE, and Belfast for bossing and Amagi, Shouhou, and Minneapolis for clearing. All level 110, and manual control. I don't think it's possible to auto that star condition, but manual went smoothly enough. Will probably revert to full fleets from here on out so I can just auto it.
