Now that I have got 15w40 and the 10w30 will be picked up tomorrow, how much 15w40 should go in for the trip to Cali? Remember its like 3000 miles round trip and will be a great way to rinse the auto rx.I was thinking maybe half and half in my 5 quart system. They are both delo 400 so I don't think there is any chemistry difference there.But, the Lincoln specifies 5w30 for all temps,probably CAFE influenced.I guess you could call it a slow revving V8, so maybe I shouldn't bother with the 15w40. Maybe all 15w40 would be best for the trip, I don't know.3 people lots of luggage and driving all day 4 days in a row then it gets parked for three weeks(maybe dry start problem there) and then 4 days back.I think the 40c viscosity is like 115 so it would be pretty thick on start up each morning and after those 3 weeks.Maybe I ought to back off to just a quart or two of the 15w40. I figure I should get some in there as I feel the 10w30 will shear quite a bit.If 600 miles on there supreme sheared as bad as it did on this engine I would surely be doing it a favor with some amount of 15w40. What I really can't understand is the blind defendance of 5w30 on the crown vic and town car websites,but maybe with ford recommending 5w20 for this old engine they will defend that too Anyway, thanks for all the help so far!