Exxon MobilBerkshire's purchase of shares of Exxon Mobil ( XOM) has been generating a lot of buzz in the last few days -- but one detail that most are getting wrong is the idea that this is a new position. It's not. Berkshire actually picked up most of its 40 million share stake last quarter, even though the SEC allowed Buffett and company to keep the move confidential as they accumulated shares. "Only" 8.85 million shares were added to the position. >>5 Stocks Poised for Breakouts That's not the first time Buffett has secretly bought shares of Exxon -- Berkshire did the same thing back in 2009. But what's interesting is that you didn't need to see the 13F filing to know what was going on. Warren Buffett has been singing XOM's praises in Berkshire shareholder letters since last year. Exxon Mobil is an integrated oil and gas supermajor with more than 18.2 billion barrels of oil equivalent in its proven reserves. That's enough scale to make Exxon the world's biggest refiner and one of the biggest makers of processed commodity products. In recent years, Exxon has been focused on reducing production costs and shifting its product mix more towards nat gas, which has helped to solidify the fortress-like double-digit margins that XOM currently sports. It's not surprising that Buffett likes Exxon -- the firm has a strong history of returning value to shareholders, thanks to a 2.6% dividend yield and a share buyback program. With a low earnings multiple priced into shares and around 5% of the firm's market capitalization covered by cash and investments, XOM is looking cheap too. A technical turnaround in shares since early October rounds out the picture in Exxon; you could do a lot worse than to follow Berkshire Hathaway into this stock.