They have screw-on caps. Or at least, every 300V can I've ever seen has had one.
This is going to be wordy because I don't have pics. But if you have a can in front of you as you read this so that you can follow along, it might make sense.
The cap basically has two D-rings on top. Flip them up and they become little handles. Pull up on them and you'll see the cap is actually screwed onto a spout that basically telescopes out of the can. Once you've pulled the spout all the way out, unscrew the cap. Beneath it, there should be a little seal with a ring you can use to pull it out, like in orange juice cartons or something. Pull that out and pour out your oil.
When you're done, screw the cap back onto the spout and press the spout back into the top of the can.
Two occasional downsides I've observed with this design:
1. The cap doesn't always screw on tightly enough before it jumps a thread, so it can be hard to get a good seal.
2. Occasionally, pulling the spout and pushing it back in repeatedly can cause the plastic to tear. At that point, it's doomed to leak. I'm sure it can be fixed somehow though.
#1 should be fixable if you just tape the cap down after it's screwed on and the spout is pushed back in.
#2 is very rare and easily avoidable (don't be rough with it).
