By Christian Berthelsen 

U.S. oil prices declined to a new six-year low on Friday as investors bet that a change in Saudi Arabia's leadership was unlikely to alter the kingdom's policy of continuing to pump crude despite falling prices.

A rally in the dollar also weighed on prices, and a closely watched gauge of China's economic activity continued to signal contraction, even as it showed slight improvement, leading some investors to fret about lower demand for oil.

The U.S. benchmark ended the day down 72 cents, or 1.6%, at $45.59 a barrel, a six-year low, after rallying as much as 3% late Thursday after the announcement of King Abdullah bin Abdulaziz al Saud's death .

The global Brent benchmark ended the day up 27 cents, or 0.6%, at $48.79 a barrel, after gaining more than 2% late Thursday.

Both contracts lost ground for the week, with the U.S. one falling 7.2%, much of that after U.S. oil inventory data on Thursday showed an unexpected increase of 10.1 million barrels in the week ended Jan. 16.

Even as oil prices have plummeted more than 55% since last June amid rising global supplies and weak demand, Saudi Arabia has kept output steady in a bid to protect its market share rather than cut production to shore up prices. Traders speculated that stance could change after King Abdullah's death was announced late Thursday, but the new king quickly made clear he would maintain existing policy and keep Saudi Arabia's oil minister, Ali al-Naimi, in place.

Saudi Arabia, the world's largest oil exporter and the second-largest crude producer after Russia, has set the tone for policy by the Organization of the Petroleum Exporting Countries, which failed to agree on a plan to reduce output at its November meeting and isn't scheduled to meet again until June.

"People quickly assessed the changeover in leadership in Saudi Arabia right now does not portend any change in their oil policy," said Eugene McGillian, senior analyst with brokerage Tradition Energy in Stamford, Conn.

Meanwhile, the dollar continued to climb Friday, extending a rally since last spring that has seen it gain 17% against a basket of global currencies in the ICE Dollar Index. Analysts cited the dollar's continued gains Friday as a factor in weakening oil prices; a stronger dollar weighs on oil by making it more expensive for buyers using foreign currencies.

And in China, the preliminary HSBC China Manufacturing Purchasing Managers Index rose to 49.8 in January, compared with a final reading of 49.6 in December, HSBC Holdings PLC said Friday. Still, the gauge of manufacturing activity remains below 50, indicating shrinking output. China is the world's second-largest oil consumer after the U.S., and traders closely watch economic activity there for signals about demand.

In refined products, gasoline futures for February delivery gained 1.71 cents, or 1.3%, to settle at $1.3479 a gallon.

February diesel gained 0.88 cent, or 0.5%, to end at $1.6467 a gallon.

Write to Christian Berthelsen at christian.berthelsen@wsj.com