5w50 too thick?

[SecondMonkey]

I've decided to use Castrol Syntec for the next oil change in my jeep with the 4.0 inline 6, 225k+. I have some Syntec already in 5w50 and 5w30. I was thinking about mixing them 50/50 for 5w40. But then again what about running straight 5w50? I've always liked 40w oils in this engine, since it's a big old iron thing from the 60s. It has a slight piston slap, and burns/drips a little oil, so thick is good. 50 too much though?

If you couldn't see the spark plugs on this thing you'd think it was a diesel while it's idling. Heavier weights quiet it down...

 

[Gary Allan]

Try it. If you don't like it ..don't use it again. The 70's 4.2 spec'd up to 20w-50 in the right ambient. I don't know how you could make this engine either more "free revving" or "slugish" either way you go.

I wouldn't buy anyW50, myself ..but that's just me. In Oz they would be wondering why you were going so light.

 

[SecondMonkey]

Well, my main concern is probably gas mileage, otherwise a 50w would probably be great for this thing. I'm already kind of compromising on the mileage thing with a 40w instead of 30. Do you think the difference between 5w50 and 5w30 would be enough to care about?

 

[Gary Allan]

Not really. Your hit on mileage may be a bit more if you're shorting it. YMMV, but I've noted no change in economy between 0w-(barely)20 and 5w-40. I think the warm up penalty is steep on this engine ..but it's not all that efficient anyway.

 

[mechtech2]

Burning, leaking, and noises are attenuated with thicker oil.
Power, gas mileage, and cold weather starting and operation are compromised.


Mix what you already own? Why not? Maybe not perfect, but very serviceable.

 

[addyguy]

Gas mileage is not really a concern on 4.0 I-6's - it's not designed to get good mileage, never has been.

I think 5W-50 would be a good oil for it. Like Gary said, you're not really going to notice a running dfference on any viscosity....

 

[sprintman]

5W50 is the biggest selling syn oil viscosity here (Mobil 1 5W50). Pretty much universal oil.

 

[SecondMonkey]

Originally Posted By: sprintman
5W50 is the biggest selling syn oil viscosity here (Mobil 1 5W50). Pretty much universal oil.

Yeah, but you Aussies are weird.

The engine may not have been designed for great mileage, but that doesn't mean it's not a concern. I've gotten everything bettween 11mpg and 24mpg in this thing, depending on the engine tune and oil at the time.

Right now it's got 10w30 Chevron Supreme in it, honestly I can't tell any difference between thicker oils I've used...but I haven't done a used oil analysis or been keeping track of mileage either. Maybe slightly louder...