This also can be done with short options adding a decay component, contact me if you like further details on the option trade
Calendar spreads in crude are based off the cost of storage, when there is oversupply.So CLM-CLZ is the cost to store the crude for 6 months, cheep storage is about 10 cents a month and ocean storage is about 110 cents a month. So the spread should be between $0.60 and $6.60 as all the cheep storage is used then the spread must increase due to higher storage costs. So the idea is if there is more crude put into storage than expected the spread should become more negative if there is less crude then expected the spread should go back towards $0.6. So yes the spread can get smaller, that would be a bullish position on oil itself. The point I'm trying to make is that there is not a normal but it depends on the amount of crude in storage and storage costs. Currently crude costs between $0.5-$0.8 a month to store.