Give it a clean and scrub with WD40.
Then lube with Engine oil.
WD40 is OK (just did some research), but I'd be careful about using some solvents to clean the chain. They can damage the O-rings on the chain.
The O-rings seal in the lubricant for each link for the life of the chain. All you need to do is provide a little lube for the rollers against the sprockets. I suggest just use aerosol chain lube sparingly every week or two / few hundred km. Maybe a bit more frequently if you use the bike in the rain. It is best to lube the chain at the end of a ride, while it is still warm.
O-ring chains are a godsend, that require little maintenance. If you have to clean them under normal conditions you are probably using too much oil / lube (or being too finicky about appearance). They last 2 or 3 times as long as ordinary roller chains, which do require a lot of attention and adjustment to get the best out of them.
The "stretch" of an ordinary chain is from wear on the pins in the links. By sealing in the lubricant on an O-ring chain that wear is eliminated. Once you have to start having to adjust an O-ring chain frequently it has come to the end of its service life.