NYMEX crude for May delivery was down 23 cents, or 0.3 per cent, at $ 66.84 a barrel at 0148 GMT, after settling up 25 cents on Thursday.
London Brent crude was down 25 cents, or 0.3 per cent, at $ 71.77 after settling down 4 cents.
Both contracts are set to rise around 7 per cent for the week, the biggest weekly gain since July.
A global oil stocks surplus is close to evaporating, Opec said on Thursday, citing healthy energy demand and its own supply cuts while revising up its forecast for production from rivals who have benefited from higher oil prices.
Opec and its oil producer allies are poised to extend their supply-cutting pact into 2019 even as a global glut of crude is set to evaporate by September, Opec Secretary-General Mohammad Barkindo told Reuters in an interview.
Oil prices jumped on Wednesday to their highest level since late 2014 after Saudi Arabia said it intercepted missiles over Riyadh and US President Donald Trump warned of military action in Syria, both of which raised concerns about possible supply disruptions.