SEOUL, Oct 6 (Reuters) - South Korea's SK Lubricants Co Ltd said on Monday it has started commercial operations at a 330 million Euro ($413 million) lubricant base oil joint venture in Spain with Repsol.

The plant has a capacity to produce 13,300 barrel-per-day (bpd) of lubricants base oil, according to SK Lubricants, which is owned by SK Innovation Co Ltd.

SK Lubricants holds a 70 percent stake and Repsol has a 30 percent stake in the venture.

The new plant will take SK Lubricants' total lubricant oil output to 70,800 bpd, or 3.5 million tonnes per year (tpy), to make it the world's third-biggest lubricant base oil maker behind Exxon Mobil Corp and Shell, the statement said.

SK Lubricants also has plants in South Korea and Indonesia.

South Korean refiners are lifting output of lubricant oil, the feedstock for engine oils and greases, in a bid to offset steep refining losses, although planned production hikes threaten to erode margins.

SK Innovation also owns the biggest South Korean refiner, SK Energy Co Ltd. Last month, another South Korean refiner Hyundai Oilbank completed a 650,000-tpy lubricants base oil plant with Shell Petroleum Co, part of the Royal Dutch Shell group. (1 US dollar = 0.7988 euro) (Reporting by Meeyoung Cho; Editing by Ed Davies)