NEW YORK -- Billionaire investor Warren Buffett's company has amassed a stake worth nearly $4.5 billion in Phillips 66 more than a year after trading a chunk of its holding in the oil refiner for a chemical business investment.

A regulatory filing says Berkshire Hathaway (BRK.A - Get Report) has accumulated about 58 million shares, which amounts to more than 10% of the Houston company's stock.

In late 2013, the Omaha, Neb.-based, company agreed to trade about $1.4 billion of its Phillips 66 (PSX - Get Report) stock for one of the refiner's businesses that makes additives to help crude oil flow through pipelines.

Many investors watch Berkshire Hathaway filings closely because they like to copy Buffett's moves, given his successful record. The Oracle of Omaha, who turns 85 on Sunday, has been running the company for more than 50 years, a period in which he has built a net worth of more than $70 billion and pushed Berkshire's share price up to hundreds of thousands of dollars.

Earlier this month, Berkshire announced that it would pay $37.2 billion for metal-component manufacturer Precision Castparts (PCP - Get Report), a deal whose size eclipses even the Omaha, Neb.-based conglomerates purchase of GEICO, Burlington Northern and Dairy Queen.

Phillips 66 shares closed at $77.23 Friday and have advanced about 8% so far this year.

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