Warren Buffett is still bullish on Wells Fargo (WFC) and American Express (AXP).

The Oracle of Omaha, whose Berkshire Hathaway conglomerate owns the biggest stake in both companies, says he's likely to buy more of their shares in the future.

"Mae West had it right: 'Too much of a good thing can be wonderful,'" Buffett wrote in a letter to shareholders published Friday.

Buffett upped his stakes in both companies last year. Berkshire bought additional shares of Wells Fargo in 2012 that increased its ownership of the nation's fourth-biggest bank by assets to 8.7% as of Dec. 31, up from 7.6% a year earlier.

Stock buybacks by American Express boosted Berkshire's stake in American Express to 13.7% as of Dec. 31, compared with 13% a year earlier.

Berkshire also owns 4.2% of U.S. Bancorp (USB), as well as stakes in BNY Mellon (BK) and M&T Bancorp (MTB).

Buffett used the latest version of his annual missive to sound off on Generally Accepted Accounting Principles, which Buffett says can distort earnings at some companies, including Wells Fargo.

"The earnings that Wells Fargo reports are heavily burdened by an 'amortization of core deposits' charge, the implication being that these deposits are disappearing at a fairly rapid

clip," Buffett wrote. "Yet core deposits regularly increase. The charge last year was about $1.5 billion. In no sense, except GAAP accounting, is this whopping charge an expense."

Deposits at Wells Fargo increased 8.4% in 2012 from a year earlier, to $945.7 billion.

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