My 1996 F-150 now with 217K on the oringinal 302 V-8 has used only Motorcraft 15W-40 for the past 117K. From new to 100K I used Motorcraft 10W-30. Always used a Motorcraft FL-1A filter. My question is I have always changed my oil at 3K intervals up until now. I would like to start extending the drains to 5K or better yet 6K. Would the present 15W-40 be ok to use or do you guys think there would be a better oil to use. The truck gets the snot run out of it with heavy stop and go, heavy loads, dirt, dust, and lots of full throttle blasts. Never had the engine apart, not even valve cover gaskets. Any oils out there could do a better job with the extended drain idea?