Most modern engines require low viscosities (a measure of an oil's resistance to flow) for multipurpose driving. Oils with a 30W grade viscosity are more lightweight, contain more friction-reducing additives to improve fuel economy and allow more efficient engine start-up even in cold conditions. Heavier, thicker oils, such as 40W oils, are more suited to older engines driven in hotter climates and at sustained speed, such as highway driving, rather than short, stop start style driving.
Forty-W oils are thicker and more viscous than 30W oils. The W indicates their "winter-grade" viscosity. Forty-W oils are less runny and do not travel as easily around the engine. Thicker oils do not progress through the engine's components as smoothly as 30W oils, but their viscous nature enables them to withstand higher temperatures, and resist wear and tear and chemical disintegration. Viscosity ratings range between 0 and 50W.
Thicker, 40W oils increase the oil pressure of the engine by their very nature, as opposed to the lighter, more flowing 30W oils. But, lighter, more flowing oils cause less dry-running in the engine, and therefore less wear and tear.
In high-mileage engines, 40W oils create a lower oil consumption than 30W oils, which is less wearing on the engine, requires less frequent oil changes and top-ups and is less expensive in the longterm.
Neither 30W nor 40W oils are really ideal for high-temperature driving conditions, although the 40W oils are the more protective of the two in this case. Straight 30W and 40W oils are not suited to cold temperatures either, because they are too thick in general. Even the thinner 30W oil needs additional work to help it function at an optimal efficiency in low temperatures. They are too thick to provide adequate lubrication for start-up. However, in this case, the 30W oil is a slightly better performer in cold weather. Synthetic oils overcome these issues, but are more expensive.
Often, 30W motor oils contain a slightly higher quantity of viscosity-improving additives to create a thinner texture. These additives have their drawbacks. In high temperatures and pressures they "shear" or break apart, and can create a detrimental sludge, which reduces the engine's performance. Once the oil has begun this process of degrading, the rating changes from a 30W oil to a 20W or 10W oil.