The United States Oil Fund (NYSEArca: USO) is up more than 13% over the past month, but some traders are not waiting around to see if the largest futures-based oil exchange traded product extends its bullish way.

Even with West Texas Intermediate futures up more than 20% this month, traders have pulled more than $300 from USO, positioning the ETF for its first monthly outflows since September, reports Moming Zhou for Bloomberg.

Outflows from USO come at a curious time. On April 6, the ETF notched its best intraday performance since February. Earlier this month, USO wrapped up the tenth-best four-day run in its history. [Big Things for a big Oil ETF]

Other oil ETFs have recently been on the receiving end of inflows. For example, PowerShares DB Oil Fund (NYSEArca: DBO) has added nearly $117 million in new assets over the past month, more than any other PowerShares ETF over that period, according to issuer data.

DBO tracks a rules-based index of futures contracts, following a so-called Optimum Yield strategy. The ETF can hold contracts as far out as 13 months and dump contracts at any point in an attempt to limit the negative effects of contango.

ETFs such as USO control a significant portion of the oil futures market. USO, which tracks West Texas Intermediate oil, held 57,956 NYMEX May crude contracts as of March 30, compared to 7,151 front-month NYMEX crude contracts back at the end of September, reports Geoffrey Craig for Platts.

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