Essentially, the heavier, thicker, or more viscous that the oil is, the better it lubricates (generally). It leaves a heavier film adhered to the surfaces being lubricated. This reduces friction. But, that means that a differential operating in 75W-140 oil will have fractionally less locking effect than the same unit operating in 75W-90. This difference is not night and day – the changes are subtle. But if you’re fine-tuning a race car, it does offer one more knob that you can turn. By the same token, lubricant formulation and additives used have a similar bearing on the matter.