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06 forester x
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Discussion Starter #1
OK guys, so recently i've been filling up alot more than normal. no extra driving i have fairly new spark plugs, clean air filter, and properly inflated tires. the only thing i can think of is when changing my oil recently i accidently grabbed 5w 20 instead of the normal 5w 30. but when reading up on oils on the net and the fozz's owner manual i got contradicting info. the manual says lower viscosity= better MPG and several websites said the opposite. is my mistake in oil weight causing this drop in mpgs? like i said its the only thing that's changed. any answers would be greatly appreciated.
 

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2008 LL Bean (4EAT)
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4,785 Posts
... recently i've been filling up alot more than normal... i accidently grabbed 5w 20 instead of the normal 5w 30... is my mistake in oil weight causing this drop in mpgs?...
No. Do the math and let us know the actual mpg for at least two tanks.
 

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2004 fxt A/T
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2,567 Posts
Regardless I would get that oil out immediately! I doubt that is what is effecting your mpg's I would say MAF or front O2 before oil. With that said change your oil, especially if it is conventional. Change your oil or lower mpg will be the least of your concerns.
 

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2019 Crosstrek 2018 XT
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14,168 Posts
Not sure I would drain it out...5W-30 ends up a 20 weight oil by 1500 miles. Having said that though I wouldn't run it hard with the thinner oil.
 

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2004 fxt A/T
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2,567 Posts
Not sure I would drain it out...5W-30 ends up a 20 weight oil by 1500 miles. Having said that though I wouldn't run it hard with the thinner oil.
Exactly, so imagine what the 20w oil is after 1500 miles.
 

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2004 Forester
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46 Posts
Did you use a different brand also? Or conventional instead of synthetic?
If the oil is so thin that the minimum oil film thickness is compromised, then you may get some metal to metal contact, and thus more friction and worse fuel economy. This condition may be even worse in the heat of summer.
 

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2008 LL Bean (4EAT)
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4,785 Posts
... If the oil is so thin that the minimum oil film thickness is compromised, then you may get some metal to metal contact...
In which case he would have more worries than mileage. This is a modern car with a modern oil, in which 5W-20 will not cause metal to metal contact under any circumstances.
Most vehicle manufacturers today specify 5W-30 or 10W-30 motor oil for year-round driving. Some also specify 5W-20... As a rule, overhead cam (OHC) engines typically require thinner oils such as 5W-30 or 5W-20 to speed lubrication of the overhead cam(s) and valve-train when the engine is first started.
http://www.aa1car.com/library/motor_oil_605.htm
 
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