Brent North Sea crude on Friday rose above $51, reaching the highest level since just before Britain's vote to quit the EU that sent markets crashing.
Brent struck $51.22 a barrel -- a peak since June 22 on the eve of the Brexit referendum -- on the prospect that producers may agree to a deal to limit high output.
Later around 1030 GMT, Brent for delivery in October eased back to stand at $50.82 a barrel, down seven cents compared with the close on Thursday.
US benchmark West Texas Intermediate for September delivery was six cents lower at $48.16 after striking a near seven-week high of $48.71 a barrel.
"The prospect of co-operation between Opec and Russia to freeze output (around current levels) has been a crucial driver of recent gains, although the market has pulled back slightly today based on movement in the dollar," said Dorian Lucas, analyst at energy research group Inenco.
Falls in the US currency this week have helped to boost demand for dollar-denominated crude. A recovery for the dollar Friday has had the opposite effect.
Prices have meanwhile roared back to enter a bull market since the start of August, rallying more than 20% from below $40 a barrel.
Members of the Organisation of Petroleum Exporting Countries and their rivals outside the group are to meet informally next month in Algeria, and there have been hints their talks could include ways to tackle an oversupplied crude market despite signs of the glut coming down.
Prices have won support this week also from official data showing falls in US commercial stockpiles of both crude and gasoline.