If you look at your spreadbetting account near the OIL prices you should have an information button or tab somewhere to tell you your minimum trade sizes and margins etc This should give you trade times also. Then, you will be able to make more informed decisions when you place a trade.. It will save on sleepless nights should you go and open something near to the end of the trading session and have to worry about it until you can next trade/sell whatever!
One word of caution regarding oil trading. Oil trading can be fun but unless you’re careful it can stop you out in a second or worse it can rack up huge losses. Not for the fainthearted.