Lehman Brothers

analyst Paul Cheng downgraded his ratings on five independent oil refining companies, changing his position from an overweight stance to an underweight one. Why the sudden flip from bull to bear?

Although it would appear as if the oil refiners would be set for short-term bullishness because of record margins, the Lehman analyst said that a closer look at the sector reveals intermediate-term weakness -- especially if the U.S. economy continues to cool. Cheng specifically cited rising production rates, strong gasoline imports and the possibility that a slowing economy could keep American vacationers indoors and off the roads this summer.

Cheng reduced his rating on

Frontier Oil

(FTO)

,

Sunoco

(SUN) - Get Report

,

Tesoro Petroleum

(TSO)

,

Ultramar Diamond Shamrock

(UDS)

and

Valero Energy

(VLO) - Get Report

, dropping them from the best rating, strong buy, to the third-best rating, market perform.