I Drove My 87 Dakota 3.9 off the lot in 1987. Every oil change has always been 5 quarts with a new filter. I recently purchased a 1990 Dakota with the same? 3.9 V-6. So I change the oil and dump in 5 quarts with a new filter. Check the oil later on the stick, and its way to full. Get the 1990 owners manual out and it says 4 quarts with filter. Why the difference?