there was a short period (can't remember exactly when) when the cost reduction guys got the to cooler The result was erosion of the inner plates causing oil and water to mix as previously stated. Early ones are fine and late ones are fine, but I have no idea of the break point. I'd probably replace as a matter of course.
No real need to go to an air/oil cooler as the performance of the water/oil cooler is fine, but some people think differently
the reputation carried on to PX Ranger that had a similar oil/water cooler but a new design from a new supplier. AFAIK no Ranger ever had this issue