Keep in mind that even though you pull all three oil plugs there is still oil remaining in the sump pan. Not that its a lot but unless you pull the pipe plugs also there will be some left in the sump galleys. It could be you last oil was just dirtier then usual from the environment. The only other thing left in there to dirty the oil is clutch fiber material say for some reason yours was slipping which will wear the fibers and burn the oil. Other then that I think you will be OK just keep a eye on her and do as you already planned. I never used the 10w40 as when I used OEM oil they were still recommending Mobil 1. since then I used turbo diesel oil and Joe Gibbs racing oil. I change every 5000 miles or when I open the engine up.