Never, it’s just unique to the choppers I have been in.
I get to tour the local Army base’s Chinooks every year in September. I have asked the crew chief(s) several times over the past 19 years why the machines always smell of hydraulic fluid/oil. The answer is invariably some version of - It’s their nature. lol...In helos I am told that many things are treated that way that would freak out a fixed wing person.
I was watching a fire fighting helo get it’s tranny fluid changed one day and the A&P pulled the plug on the pan it came out with a string of metal shavings about 2” long stuck to the magnet. I commented that it was time to take it out of service. The guy said “No, as long as the pieces come out the hole it’s good to go!” YIKES!