I wonder if there are differences in the various...grades...of pot? Is some oil more potent than others?
That depends on the plant. They aren't created equally. Some plants have a higher cannabinoid content than others. And even then, it's not an exact science.
There are basically two types of pot plant, Sativa and Indica. Indica is what Rick Simpson likes to use because it's highly relaxing. Sativa tends to be more invigorating, and somebody recovering from serious illness doesn't need to be up, running around. They need rest. He reccomends Sativa more for conditions like depression.
Both will take care of cancer and serious illnesses equally. It's just the relax/invigorate effect that makes the difference. A LOT of medicine growers/makers like to use a Sativa/Indica hybrid, for the best of both worlds.
If you're buying the plant material from somebody else, whoever's selling it probably won't be able to tell you if it's Indica or Sativa, or what the THC content is. That's a crap shoot. Still worth doing, though, if that's all you've got, because it's going to have SOME medicine in it, no matter what.
But if you're looking to grow your own, it's pretty easy to find out what you're getting. Just check the website you're buying your seeds from, and only buy from one that can tell you not only what the plant is (Indica or Sativa) but also what the THC/cannabinoid content is. It's often expressed as a percentage. 20%, for example, is pretty darned good. 25% is better. There are plants that grow well indoors, others grow well outdoors, and plants can range from a foot tall that you can grow anywhere, to 7-10 foot tall towering plants. And there are plants that mature in 6-12 weeks, so somebody waiting for medicine doesn't have to wait forever.
I just Googled "Buy Indica" and the very first site that popped up in search has all that information, and pretty reasonable prices. And they ship everywhere.
As for growing, some people will tell you that you need an expensive gardening light assembly, but that's rubbish. You can get a 20 dollar bulb from Walmart, as long as you make sure it's a full spectrum light. Use a good, rich, ph balanced soil, and you should be good.
Then, when it buds, harvest the bud and make the oil.
It's a lot of information, but it's easy to follow. Plus, I'm usually around to answer any questions anybody has.