I've had the tan film too, large parts of the machine at least, it's oil mess not cleaned up. Decades later it darkens. Metal bars and parts usually shine up with a type of metal polish, but it's cumbersome getting it done inside the machine. Any grit left behind needs to come off too. It's a must to shine up the neeedle- and presserbar though , makes the machine look so much cleaner :- )
I have noticed my machines gradually look cleaner inside, but it takes a couple of years. It's the repeated cleaning and oiling, the odd wiping off now and then. Sometimes I'm a bit more thorough than others.
This is a nice blog post related to the 401. She will talk you through a lot of the tricky parts in
this video; and she is very good with these machines.