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Is this oil going to hurt my engine?

ultimatebob

Lifer
Jul 1, 2001
23,901
1,667
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I have a 2011 Chevy Cruze Eco with 105,000 miles and a pretty nasty oil leak due to a leaking rear main seal. It's a piece of crap, and my local stealership wants $2,000 to replace the seal. I told them to pound sand, obviously. I can get a lot of oil for $2,000.

Thanks to the leak, I lose about a 1/4 of a quart of oil every 2 or 3 weeks. I always keep a jug of oil in the trunk to keep it topped off every oil change.

I noticed that I have 6 quarts of new Mobil 1 "High Mileage" Synthetic 10W-40 motor oil in my workshop from the prior owner, which I was wondering if I could use in the Cruze in a pinch. It looks like quality stuff, but the recommended oil for the Cruze is a 5W-30 synthetic blend.

I tried Googling the question of using this oil in my Cruze, and got back mixed results. Their results came down to the same conflicting statements:

Don't do it:

1) Chevrolet and their oil suppliers all say that you should Only use the recommended oil from the manufacturer. OK, sure, but I'd imagine that their lawyers and their marketing department make them say that.
2) A few mechanics said that using a thicker oil than what's recommended could cause lower gas mileage, and/or shortened engine life due to increased wear.

Doesn't matter:

3) Many mechanics said that putting 10W40 motor oil in a car that's supposed to have 5W30 in it isn't a big deal, since they are almost the same viscosity. Unless you live in an area with extreme weather, it shouldn't matter.

Do it:

4) A few mechanics actually recommended using thicker oil in high mileage engines (like the one I have), as the engine now has looser tolerances due to wear. The "high mileage" oils also supposedly have additives that help to condition the engine seals and reduce leakage. Again, I think that their marketing department made up that last statement. Besides, the seals in my engine were beyond "conditioning" months ago.

Either way, I don't think that it really matters what kind of oil I put in this car at this point, since this oil will just be adding to the oil spots on my driveway before it can do any serious damage.

What do you guys think?
 

lxskllr

No Lifer
Nov 30, 2004
54,714
4,704
126
You're a yankee, right? If so, I'd wait til warmer weather to use it. I'm with the thought of using heavier oil in aged engines, but I'd be leery of doing that in a small engine in winter.
 
  • Like
Reactions: Yuriman
Jun 19, 2004
24,135
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Check with a regular mechanic not, the dealership. High end for the rear main seal should be more in the $800 range.
 
May 13, 2009
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I'd use 5w30 when its time to change the oil and would use the 10w 40 for topping it off. If you're worried about it grab a cheap gallon of 5w20 and mix it half and half with the 10w40. That would give you a viscosity closer to the 5w30 that's recommended.
 

ultimatebob

Lifer
Jul 1, 2001
23,901
1,667
126
You're a yankee, right? If so, I'd wait til warmer weather to use it. I'm with the thought of using heavier oil in aged engines, but I'd be leery of doing that in a small engine in winter.
If you call southern Connecticut "Yankee" county, sure. It never gets below 0 degrees F here, though.
 

lxskllr

No Lifer
Nov 30, 2004
54,714
4,704
126
If you call southern Connecticut "Yankee" county, sure. It never gets below 0 degrees F here, though.
That's still pretty cold. You want quick flow when you start it up, and the lighter oil will give you that. The oil in the garage won't go bad over a year or so. I think it's sensible to follow factory recommendations under fairly severe conditions, then free form it when the weather moderates. You can get rid of the heavier oil spring-fall.
 

Bartman39

Elite Member | For Sale/Trade
Super Moderator
Jul 4, 2000
8,879
50
91
10-40wt oil in a beater that has over 100k miles...? Even up in the northeast I would any time and full strength at that... Just be easy on the engine when you crank it and its cold outside or keep in a garage...? Or even better plug it in if it has a block heater...? Or heck just add one if it does not...?
 

Yuriman

Diamond Member
Jun 25, 2004
5,530
141
106
105k miles doesn't seem all that high to me. Maybe for an engine built 30 years ago. I second the idea of not using that oil until it warms up.
 

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