OEM specs vs. Temp and Viscosity Charts

[wayniac]

I have a question regarding OEM specs for oil vs. the charts I see for temp and service. My wife has a dodge nitro (07) and I see that in the owners manual they specify for the 3.7 liter a 5w-20 non-synth, and for the 4.0 a 10w30/10w-40. Why would a 5w-20 be the OEM choice for oil when the other engines and viscosity charts rate thicker oils to be better protection in use under temp? This puzzles me, and its time for the first oil change, so I am asking for advice. I have a mazda MX-5 miata that I rev the snot out of and the OEM rec's 5w-20 there also, but I reasoned that its a small high rev engine so the oil may need to be a lighter weight??? Why on earth would a 5w-20 be acceptable or even preferable for the 3.7 over the other heavier weights that are specified for the 4.0? The temp charts show better protection for heavier oils, why arent they used? What should I use? We light tow, but wife works in Houston, so stop and go traffic and HOT enviroment for operating condition. Thanks for advice!

 

[MrWideTires]

In my house we have a Dodge Ram with the 5.7L hemi, I drive it hard and it tows, all with 5w20 in Florida heat... 100k+ miles with no problems

 

[d00df00d]

Originally Posted By: wayniac
I have a question regarding OEM specs for oil vs. the charts I see for temp and service. My wife has a dodge nitro (07) and I see that in the owners manual they specify for the 3.7 liter a 5w-20 non-synth, and for the 4.0 a 10w30/10w-40. Why would a 5w-20 be the OEM choice for oil when the other engines and viscosity charts rate thicker oils to be better protection in use under temp? This puzzles me, and its time for the first oil change, so I am asking for advice. I have a mazda MX-5 miata that I rev the snot out of and the OEM rec's 5w-20 there also, but I reasoned that its a small high rev engine so the oil may need to be a lighter weight??? Why on earth would a 5w-20 be acceptable or even preferable for the 3.7 over the other heavier weights that are specified for the 4.0? The temp charts show better protection for heavier oils, why arent they used? What should I use? We light tow, but wife works in Houston, so stop and go traffic and HOT enviroment for operating condition. Thanks for advice!

There are two parts to this.

The first part is that oil thins out as it heats up. If the engine runs hotter, you need an oil that is thicker to start with so that it doesn't thin out too much. That's what the temp chart is showing you. It's not about protection; it's about heat.

The second part is how the engine is built. For example, if one engine has tighter clearances or a more powerful oil pump than another, it might prefer a thinner oil.

If the 4.0 specs thicker oils than the 3.7, it could be because the 4.0 is expected to run a lot hotter, or it could be because the two engines are just made differently.

The one thing you can rest assured of is that protection and viscosity don't go hand-in-hand like they used to. You could redline, race, tow, or haul all day long on a 0w-10, provided the oil temps stay low enough and the engine is designed accordingly.

 

[mechtech2]

Often cars have 5-20 or 5-30 on the oil cap, but those ancient charts are still printed in manuals.
The charts always spec heavy oil.

 

[wayniac]

Thanks! I guess that I still put the viscosity and temp charts in the catagory of "protection" in my mind. Thanks for disconnecting. I will go with the OEM rec. 5w-20 and let my mind rest easy.