Prices have been fluctuating since entering a bear market last week, falling more than 20 percent and closing below $40 a barrel for the first time since April.
Initially recovering as US commercial crude inventory data showed an unexpected draw in gasoline, prices tumbled again later in the week after robust hiring numbers from the US government pushed the dollar higher.
A stronger greenback typically makes dollar-priced oil more expensive for consumers using other currencies, denting demand.
Weekly rig count data released Friday also showed that US drillers added seven rigs that week, up for the sixth straight week and indicating higher production activity amid an oversaturated market.
At around 0630 GMT, US benchmark West Texas Intermediate for delivery in September was up 28 cents to $42.08 and Brent crude for October gained 29 cents to $44.56 a barrel.