Should I run 10w40 in my '78 300 I6? I have heard of some guys switching to 10w40 when their engines a higher miles on them. Why? I know this is probably pretty basic stuff, but I was wondering what the reasoning behind this is. Thanks.

 

i run 15W30 Rotella Diesel oil in my truck
it is warm enough where i live that i can use it year round and it gives better lube than 10W30.
I also like tha additives to keep the inside shiney

 

Think of 10W40 as two different oils. 10W is the base oil which is what they start with, then add cleaning agents, additives, and viscosity modifiers(LOTS OF THESE! this is where you get the 40 from) and it'll thicken up as it warms up. HOWEVER GM Banned it from their vehicles about 20-25 years ago as the ring lands were getting gummed up from the viscosity modifiers and they were seeing a lot of engine damage. FORD and Chrysler didn't have that problem (or at least didn't identify it as such) and it's still used as a secondary oil weight in older vehicles.

Personally I use 30W(SUMMER), 40W (SUMMER) or 10W30(WINTER) in my 4.9l depending on the time of year. Where you're at in AR you can pretty much use whatever you like, but you might want to stay away from the 5w30 or 10W30 in Summer. If you got a lot of miles and you run it all day the pressure tends to stay low. I myself like a fair amount of pressure.

Also Rotella Diesel Oil is ok if the engine has been freshened up recently(under 50,000). It's got a lot more cleaners and if your engine isn't inside it may clean it too much and you'll start having problems with clearances, etc.

Your choice.