The design basis is that it is an oil cooler, not a fuel heater.
the contact times for a plug of each fluid are not necessarily the same
That is why the oil takes a convoluted path around the tubes, diverted back and forth by baffles, to ensure a (relatively) long residence time in the cooler and thus maximum cooling.
The fuel is supposed to go straight through the tubes and out the other end, with minimum restriction to the fuel flow. Very little heat from the oil passes to any particular molecule of fuel, but that doesn't matter because there is plenty more cold fuel coming along behind.
Of course, the guy who designed it that way didn't know that lumps of ice were going to be thrown at it.
If you swap the flow paths between fuel and oil, the result will be warmer fuel and much less well-cooled oil.