Why not 5W40?
 

Well, like the trailblazinfool I am I bought some Mobil 1 5W40 before I checked the owners manual. The manual states DO NOT use 5W40 or anything other than 5W30 or 10W30. I am used to my older cars that can take alot of variations depending on temp ranges.

The truck has been serviced at the dealer every time up until now. It's my wife's primary and she wanted to just let the dealer do it the first few years.

Funny thing she has asked for Mobil one ( or at least synth) and several times they only changed it with conventional and she always asks when paying the bill and catches it, as the synth change is higher cost. They have had to go back and change it again... but I wonder if they really did or just told her they did?

Anyway, I found some Mobil one SUV/Truck oil in 5W40 and it was half price... I am wondering if it would really screw anything up. My gut tells me no... 62,000 and still alot of heat left in TX for summer... I would change it back to 5W30 in Nov or Dec.

Thanks for any replies.

RE: Why not 5W40?
 

A heavier weight oil may be useful in worn or high performance engines that have had bearing clearances set on the loose side, but in a stock production engine a heavier weight oil will not flow as well to rod and main bearings as the lighter weight will. To a degree you will be starving your bearings of oil. Will the engine survive, most likely it will but why not use what is recommended for the engine. Ford is recommending 5W-20 for some of their engines.

RE: Why not 5W40?
 

Thanks for the reply... I am not new to engines and mechanical theory, I just find it hard to believe that a multigrade 5W40 VS 5W30 would be too thick in hotter climates in the summer months. I spoke with someone else about it that said GM and other manufactures do it to get the MPG higher for the CAFE.

RE: Why not 5W40?
 

Most of the engines I built for dyno testing to destruction, "wide open throttle under load until it broke", used 5W-30 oil, both synthetic and mineral oil. The usual failure was the fatigue of a hard part, not a lubrication failure. I use the recommended 5W-30 Mobil 1 in both of my vehicles and I made that decision based on test data. As far as the recommended oil viscosity for your vehicle, alot of testing went into that recommendation so I don't try to second guess the accuracy of it. As I mentioned, your engine will likely be fine but keep in mind that the heavier the oil is, the less able it is to properly flow into real tight bearings. By using a synthetic oil you will also slightly improve your fuel economy.

RE: Why not 5W40?
 

5W-20 is used PURELY for gas milage for Ford. The Ford Explorer calls for 5w-20 yet the Mountaineer that uses the EXACT same motor, all parts are interchangable and has the same engine code, calls for 5w-30. My S10 uses 5w-30 or 10w-30 I've gone to amsoil15W-40 desiel oil with out a problem. The reason for doing so was for higher oil pressure during cold days.