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Old 04-06-2011, 11:35 AM   #1
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What octane gas what weight oil?

What octane gas and what weight oil should we feed or TJ's with 4.0L engines for everyday driving?

I have been filling it with 87 and using lucas fuel treatment to clean it. I seafoamed it once.

I use Mobil1 Full Synthetic 5W-30 and Fram or royal purple filters.

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Old 04-06-2011, 11:40 AM   #2
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87 is all you should ever need in a stock 4.0L engine... As far as oil you are fine there with the 5w-30. 10w-30 is also fine and I believe what most people use.

Fram however should never be used. Surely someone around you sells a mopar filter, wix, or purolator.

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Old 04-06-2011, 11:42 AM   #3
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87 octane is as much octane as the engine can utilize, there would be zero benefit to paying more for a higher octane.

No need for any aftermarket fuel treatments either, today's gasolines all come with more than enough fuel injector and fuel system cleaning additives to keep things sparkling clean. Dirty fuel injectors are pretty much a thing of the past. That wasn't always the case for the first ten or so years that fuel injection started showing up in our vehicles. Gasoline manufacturers didn't include enough cleaners so clogged fuel injectors were fairly common... but they since fixed that problem and you won't find a U.S. gasoline without enough cleaning additives to keep things clean.

In fact, I used to have 20+ new car/Jeep clients here in San Diego and I used to make it a point to talk with the mechanics when I had time. I asked them all about dirty fuel injectors. Only the older mechanics had seen them and all said they hadn't seen a clogged injector in years. None of the younger mechanics had ever seen a truly dirty/clogged injector.

Save your $$$ on those fuel injector cleaning additives, they simply aren't needed any more. Despite what the counter guy at the local auto parts store will try to convince you of.
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Old 04-06-2011, 12:39 PM   #4
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I actually did my own comparison last summer between 87, 89, and 91 octane gasses. I ran each for three weeks, totaling roughly 900 miles on each. I made sure to drive the speed limit, and take the same route to and from work each day. With the 87, I was averaging 12.8mpg. 89 = 13.1, 91= 13.8. It's not much but it did make a difference.
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Old 04-06-2011, 12:48 PM   #5
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I actually did my own comparison last summer between 87, 89, and 91 octane gasses. I ran each for three weeks, totaling roughly 900 miles on each. I made sure to drive the speed limit, and take the same route to and from work each day. With the 87, I was averaging 12.8mpg. 89 = 13.1, 91= 13.8. It's not much but it did make a difference.
I'm surprised that the 89 was only 0.3 mpg higher than 87 (but 0.7 mpg lower than 91), considering that the pump just blends 87 and 91... maybe the blend is not really 50/50. In any case, 87 is all you need, and I don't think such a slight improvement in mpg is worth the extra cost.
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Old 04-06-2011, 12:55 PM   #6
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i use 87 octane with mobile one synthetic. then about every three months i poor a can of engine clearner in my gas tank it really does make it preform alot better
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Old 04-06-2011, 01:13 PM   #7
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I use Mobil1 Full Synthetic 5W-30 and Fram or royal purple filters.
Depending on where you live 5W-30 is probably best in the Winter. Here in SoCal, I am running 10W-30 in the Spring, Summer, and the first part of Fall when the temps can still get above 100. Then I switch back to 5W-30 for the wintertime - where I live, we get a lot of sub-freezing weather, and one year it got down to around 0F.
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Old 04-06-2011, 01:16 PM   #8
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Octane has zero/nada/none/no effect on mpg. Period. None whatsoever.

What do you guys think octane is, does, and its purpose?
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Old 04-06-2011, 01:28 PM   #9
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For at least the last 10 years all major fuel stations have filters on the gas dispensers. This has stopped a lot of junk from getting in your fuel tanks. At my son's shop they don't see dirty injectors anymore. The injector cleaning kit sits idle, anyone want to buy one?
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Old 04-06-2011, 01:31 PM   #10
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Octane just makes gas more difficult to ignite in lamest terms.
It also slows the burn of the fuel.

When running higher compression, gas can sometimes be ignited just from pure compression and not from the spark plug (like a diesel) so the fix was to add octane, it helps keep gas from igniting before the engine is at TDC and the spark plug lights the fuel.

adding more octane when its not needed will do nothing but slow the rate of which the fuel burns.
too much octane can actually lower power as the fuel has not completed burning during the power stroke and will keep burning when the cylinder starts its exhaust stroke.
when a engine runs a low enough compression for 87 octane, there is NO advantage at running higher octane.
I have heard rumors that slightly higher octane then whats needed will help the engine run cooler.
thats my version and knowledge of it.
might be wrong
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Old 04-06-2011, 03:24 PM   #11
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Nicely done Mr RPM, you nailed it on what octane actually does.

A higher octane won't help the engine run any cooler, though a richer air/fuel mixture could. Today's lean fuel mixtures do cause engines to run a bit hotter.
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Old 04-06-2011, 03:31 PM   #12
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forget beating dead horses, im gonna start beating people who bring up octane/mpg ratios from now on.
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Old 04-06-2011, 04:41 PM   #13
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it is possible that the 91 will give better mileage. not from the octane but lower levels of ethanol. a friend of mine was talking to the owner of a gas station by the lake, she told him that normally premium has lower levels than regular. (some gas stations around lakes still order gas with out ethanol about 20 cents higher than normal but it does not absorb water or hurm aluminum heads)
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Old 04-06-2011, 04:49 PM   #14
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I have heard that, and i think that whole thing started when ethanol was just starting to be introduced to gasoline. Maybe premiums used no or less ethanol at the time and thats where the whole "octane give better MPGs" thing started. Because as far as I know, ethanol does give less MPGs but in turn is less harmful to the environment (even though you gotta use more of it in the long run) and is made from a renewable resource.
I drove a rental car that has an e85 flex fuel option. the MPGs would drop 5MPGs on e85 but as we know e85 is .50$ or so cheaper.

but now days all octane ratings are regulated and use the same 10% ethanol, atleast in my part of michigan. but thats all because of ethanol levels and not octane as for what MPGs you get on different grades of gas.

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