smazur

Here's the situation on my 97 E320, 130k miles...

My mechanic is keeping his eye on a slight head gasket leak that I've got and he had a cow when he learned that I was using Mobil 1 10w30 in my engine. He said changing to regular dino oil would slow the leak down somewhat as it's a bit heavier... He even said that I should go with 15w50 for best result with my older engine.

So I switched to 10w30 Mobil dino oil to start and everything is fine but I've noticed a pretty big drop in my gas mileage. I'm talking about around 50-75 miles per tank difference. In a tank of city only driving I just got 297 miles which is awful mileage. Normally I'm up around 360 miles...

Anyone care to comment on whether you think the oil type can affect it this much? Also, what would I expect if I went to 15w50?

E-Klasse

Can engine oil affect gas mileage?

Yes.

To what amount, as they say, Your mileage may vary.

Synthetics will perform better than hydrocarbon oil even with the same weight. The molecule size is more uniform so there is less internal friction and that is also why they tend to leak more.

Your engine waists energy moving the oil around as it operates and it will waist more with thicker oil.

zam2000

That sounds about right, going from 30 grade synthetic to 50 grade regular oil really take a tool on fuel comsumption. I experienced the same thing.

I once put my previous car (LexusGS300) in for oil change at independent shop. The shop put 20-50 regular oil, even though I did ask them for 10-30. The freaking owner even gave me a lecture on how 50 grade oil is better for car that pass 100K mile. MPG took a severe hit after that, and that was at a time when premium fuel was more like $1.5-2.0/gal. Imagine how mad I was.

I changed oil back to 10-30 synthetic after two gas tank, and all was back to normal.

smazur

what about this summer time gas blend that I hear about? Can a new gas blend affect the mileage?

E-Klasse

The more ethenol they blend in it, the less miles per gallon you get.