Oil fell to its lowest in six months on Monday, knocked by fresh evidence of growing oversupply and data highlighting slowing demand in China, leaving crude prices set for their weakest third-quarter performance since 2008. A Reuters survey last week showed oil output by the Organization of Petroleum Exporting Countries reached the highest monthly level in recent history in July. Brent crude oil fell $1.17 to $51.04 a barrel by 0850 GMT, having touched a low of $50.85 earlier in the day, its weakest since January 30. The price has lost nearly 20% so far in the third quarter, which would make this the biggest slide for the three months from July to September since 2008. US crude fell 75 cents to $46.37 after hitting the lowest in four months at $46.35. Front-month prices lost 20.8% in July, the biggest monthly drop since October 2008.