15/40 formulations usually come in the forms of HD Deisel oils that are overbased for passenger cars and have additives that a gasoline motor simply does not need. More is not better when it comes to oil formulations . Some call these oils Fleet oils and they effectively lower the stocking oil for a transport company that might have 15 deisels and two gasoline powered runners . Also the 15/40 might have a SG gasoline motor designation when actually the owners manual might call for a SL. Of course the newer CI-4 HD oils have lower levels of some additives . Without your car in the topic even a newer HD oil with a SL designation simply has too much phosphorous and ZDDP for the catalyst system for the car that is used normally. The spark plugs will have premature deposits along with the combustion chamber if the engine is not ran to it's limit often from what I have seen . Also gas milaege suffers using a overbased oil in a gasoline motor .The base oil it'self is usually heavier and more viscous at lower operating temperatures than the passenger car 10/40 wt little brother.
These are very tough oils and would be a great choice for that long summer trip while towing a boat or other trailer and the engine will get some extra cleaning from the use of a HD oil but full time duty in a gasoline is not desirable in my opinion or the Major Oil Company's.
[ December 30, 2002, 07:09 PM: Message edited by: deluxekitty101 ]
15w40 oils have very high TBN and are great for extended drains. Won't hurt you to try it! I'm going to use 15w40 Royal Purple/Longrider synthetic oil soon. Maybe Amsoil 15w40 synthetic in the future too if I can get it for $4.00/qt. I buy by the 4 gallon case, and that should last me a year.