The factory cooler has internal fins in the tube to help move the oil around. I couldn't see them until I cut the tube near the cooler to fit my new cooler in series with the old unit. Also the factory cooler tubes are flattened in the cooler area which helps get the oil near the tube walls. I can't comment on the factory coolers suitablity for an XJ (seems too small to me), but it's design and construction appears to be very good for a tube and fin design.
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