Liquidity is how much can be traded without moving the price. It is not volume, though the two correlate: volume is what has already happened, liquidity is what you could do next. The practical measure is the cost of a round trip, which has three parts.
The bid-ask spread is the visible part, and in front-month WTI it is usually one tick. Depth is the size available at that spread, and it is what actually constrains a real order: a one-tick market for five lots is not a one-tick market for five hundred. Market impact is what your own order does to the price as it consumes that depth, and it is the part that does not appear on any screen until you have paid it.
In crude, liquidity is concentrated to a degree that surprises people coming from equities. It sits in the front two or three months of the two main benchmarks, and it falls away sharply along the curve. It also concentrates in time: the hours when both London and New York are open carry most of the day's depth, and the settlement window carries a disproportionate share of the volume.
The consequence is that the curve is not uniformly tradeable, and a strategy that looks clean on settlement data may be untradeable where it matters. A spread between the first and second month can be done in size at a tick. The same spread between month thirty-six and thirty-seven has a settlement price every day, a chart you can draw, a z-score you can compute, and almost no market.
Liquidity also migrates in a predictable way. As the front month approaches expiry, open interest rolls to the next one, and for a few days the second month is more liquid than the first. That is why a continuous front-month series has to define a roll rule, and why the definition is not a technicality: rolling on expiry day, on a fixed calendar date, or on open-interest crossover produce visibly different histories. See roll yield.