The time oil spends in the cooler depends on the size of the cooler, not how many times it passes through it. The amount of oil going through the cooler is constant (determined by the oil pump), if 1litre/sec. is pumped into the cooler, 1l/sec has to go out the other end. If the cooler is 1 litre in size, oil stays 1 second in it. If it is twice 0,5l (double pass, same size), oil still stays 1 second in the cooler. Simples