You can replace your "6-speed" chain with any new "8-speed" chain and it will be fine. Many higher-mileage riders treat chains as disposable: replace it before the rear cogs wear enough to cause skipping. This is more important on your bike given the difficulty of finding replacement freewheels. New ones are available and usable, but they are not same as you bike's original freewheel.

Cleaning and lubing chains is an ongoing debate on every bicycle forum I've been on in the last 25 years. Not much has been settled. IMO, pay attention to the basics:
Wipe the dirt off the chain off after every ride.
Clean the chain thoroughly before lubing.
Don't over-lube.