Every quarter, many of us in the investing world eagerly anticipate the SEC filings that the world’s largest (and greatest) investment managers are required to file.

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There is no single 13F that is more eagerly anticipated than that of Berkshire Hathaway’s (NYSE: BRK-B) Warren Buffett.

The Oracle of Omaha isn’t just one of the greatest stock pickers the world has ever seen. He is also one of the most selective. He seldom makes a significant new addition to his portfolio, usually instead preferring to add to existing positions or buy nothing at all.

So when he makes a big purchase of a new stock, it’s kind of a big deal.

In the most recent quarter, Buffett revealed a rare new portfolio addition: Exxon Mobil (NYSE: XOM) — for a cool $3.4 billion. Even for Buffett, that’s a big purchase.

Investors have been scrambling to answer two questions: Why is Buffett was buying Exxon — and why now? Buffett has been familiar with Exxon for decades, so what’s changed about the company that merits a $3.4 billion investment from Berkshire?

It certainly isn’t because Exxon’s stock price has dropped:

Exxon’s shares are actually close to all-time highs, and a similar or better entry price could have been had for a long time.

The “Why now?” part of the Exxon purchase stumps me — but I think I’ve got a pretty good idea about the why.

It is all about the price of oil. I think Buffett expects it is going to rise over the medium and long term.

The reason I think that is because Buffett’s longtime business partner, Charlie Munger, said so.

Of course, Munger didn’t tell me that directly. But he did tell anyone who happened to come across a roundtable presentation that the Berkshire vice chairman participated in.

Munger’s one of the most rational thinkers in the entire investment world, and he’s almost always on the same page as Buffett.

Munger had this to say about future oil prices:

“Oil is absolutely certain to become incredibly short in supply and very high-priced. The imported oil is not your enemy, it’s your friend. Every barrel that you use up that comes from somebody else is a barrel of your precious oil, which you’re going to need to feed your people and maintain your civilization. The way to do that is to go very slow in producing domestic oil and not mind at all if we pay prices that look ruinous for foreign oil.

“The oil in the ground that you’re not producing is a national treasure … It’s not at all clear that there’s any substitute [for hydrocarbons]. When the hydrocarbons are gone, I don’t think the chemists are going to be able to just mix up a vat and create more hydrocarbons. It’s conceivable that they could, I suppose, but it’s not the way to bet. We should spend no attention to these silly economists and these silly politicians that tell us to become energy-independent.”

Exxon isn’t the only oil producer that Buffett has purchased in 2013. He also made a $500 million investment in Canadian oil sands producer Suncor (NYSE: SU). These are bullish bets on the price of oil, and Munger has told us why.

It doesn’t get any easier than this for investors. We have one of the greatest stock investors ever alerting us to a long-term trend we can profit from.

This is a fat pitch for the long term. Get exposure to oil stocks, sit back and let higher oil prices take us to investment gains.

We don’t even have to pick a stock for ourselves. Both Exxon and Suncor trade close to Buffett’s purchase price, which means we can piggyback directly on these two stock picks from the master.

Risks to Consider: Disruptive technology. Buffett and Munger may think high oil prices are quite likely, but very smart people are looking for alternative energy sources every day.

Action to Take –> Buy shares of Exxon and Suncor for long term exposure to the price of oil. Both companies generate significant amounts of excess cash flow, which are used to pay dividends and repurchase shares.

P.S. Stocks owned by investing gurus like Buffett are just one of the criteria StreetAuthority’s Michael J. Carr looks at when deciding what to recommend in his premium advisory Guru Trader. The system he’s perfected during his 26-year trading career not only screens for stocks owned by investing legends like Buffett — it actually tells him what to buy and when to buy and sell. To learn more and get Michael’s next “guru trade” in your inbox, click here.