* Wall St slumps, volatility index nears three-year high
* Oil still up 1 percent in 2018
* Seasonal drop in demand stemming from refinery maintenance (Updates prices, adds Exxon refinery maintenance)
LONDON, Feb 6 (Reuters) - Oil fell for a third day on Tuesday, swept lower by a wave of selling that hit equities, bonds, cryptocurrencies and commodities, although the crude market is in positive territory so far this year.
Even with Wall Street stocks posting their largest one-day fall since late 2011 on Monday and measures of volatility spiking to multi-year highs, reflecting heightened investor nervousness, oil has not suffered to the same extent.
Brent crude futures were down 81 cents on the day at $66.81 a barrel by 1416 GMT, but still up 1 percent so far in 2018. U.S. crude futures fell 72 cents to $63.43.
Since the S&P 500 hit a record high on Jan. 26, the index has lost 8 percent. Oil has shed 4.5 percent, while cryptocurrency bitcoin has lost half its value.
One factor that has helped insulate oil to some extent against a bigger rout is the structure of the forward curve, where the prompt futures contract is trading well above those for delivery further in the future.
"We know that speculative positions both in terms of contracts and in allocated dollars are at an all-time high. Thus a real pain-trade has not yet hit the oil market," SEB head of commodity strategy Bjarne Schieldrop said.
"Longs have not yet started to flock to the exit door. If that happens, it will make the buying opportunity even better for the oil consumers who buy oil on the forward curve."
Financial markets sank on Monday after a sharp rise in U.S. bond yields raised concern over a possible increase in inflation and potentially higher interest rates.
Oil has been caught between the opposing forces of a 1.8 million barrels per day (bpd) cut in supply by the Organization of the Petroleum Exporting Countries and Russia, and a surge in U.S. crude output above 10 million bpd.
There is also a seasonal downturn in demand, as many refineries shut for maintenance at the end of the peak-consumption winter season in the northern hemisphere.
Exxon will close the distillates unit at its 320,000-bpd Antwerp refinery in March for maintenance, traders said.
"It is, however, worth remembering that global oil demand is set to grow at a healthy rate this year, that OPEC and its 10 non-OPEC peers are impressively disciplined at keeping their quotas and that geopolitics also helps to balance the supply/demand equation," PVM Oil Associates strategist Tamas Varga said.
(Additional reporting by Henning Gloystein in Singapore; Editing by Dale Hudson)