Like in any other type of radiator, the tubes running through an oil cooler core can sometimes become blocked. This limits the flow of water through the tubes and prevents the cooler from effectively lowering the temperature of the oil. Plugged water tubes can often be fixed with a thorough cleaning. However, blocked oil passages usually require a core replacement.
The oil filter should be carefully inspected any time an oil cooler core is installed or replaced. The quality of the filter can help indicate the quality of the oil cooler core. For example, if there are small chips present in the oil filter, it probably means there are also chips in the oil cooler core. This is especially true if your system is built so oil flows through the cooler first and then the filter, as most systems are. In that case, the oil cooler core cannot be completely cleaned and will likely have to be replaced. As a general rule of thumb, if your oil filter is somehow contaminated, your oil cooler core is probably contaminated too.