Nihon - as MIserlou says - please DO let someone know. Your input may not ALWAYS be welcome depending on the psyche of the crew, but it is right to do so if you are concerned.
I recall a DC3 flight a few decades back where I told the ageing Captain that there was oil seeping out of the rivet line on No2 engine.
"**** good, he said - that shows there is oil in the ba***d"
Still - I felt better!
FYI, oil consumption is monitored on jet a/c, and the uplifts show how much is going. There are limits in the maintenance books and crews should not depart with less than a certain amount, remembering the length of the flight, of course!