Agreed, 40k is not my whole house, just a remortgage of around 40%. Agreed about the booms, but scarcity and middle east dictatorships are a sure fire bet - I would bet my life on it
could turn into your whole house quite easily - for the sake of an example as to how this could go wrong very easily
OK supposing you put up your £40k margin and go long 20 brent crude futures (assuming for this example $60K USD in margin and $3K margin per contract)
the price drops by 1 dollar, you lose $20,000
also assuming your broker hasn't actually closed your position automatically already(more than likely with most) - do you take out another mortgage to cover the additional margin you now need to maintain it?
the price drops by a couple more dollars
take out another mortgage? You've now required an additional £40K GBP ($60k) to hold the position over this $3 drop in the price of oil.
Sure you might be very very lucky and go long right at the bottom of the market, see the oil price increase by over $30 over a few months and get a 1000%+ return on your investment... in reality that isn't going to happen
Price can fluctuate a fair bit.... maybe oil will increase over your horizon of several years but can you afford to hold the position while it bounces around - would you be prepared to risk your whole house? The most liquid contract is the front month, currently that expires in December - you'll have to roll your position to the next month and so on.
So maybe you don't invest the whole £40K... maybe you just buy a few contracts and have the rest in cash to cover additional margin requirements - well you're not so leveraged now so your huge % returns aren't feasible any more.
Maybe you can buy a load of call options... but there is a reasonable chance they'll expire worthless.
I think if you're going to take a long term view on the price of oil then using lots of leverage to do it is a bit silly, the price can bounce around all over the place... If you want to speculate on these price fluctuations over a shorter period of time then perhaps it is feasibly to make use of leverage... problem is there are rather a lot of fairly sophisticated firms doing that and you're not likely to beat them at this game.