Oil Weight vs. MPG

wally28 · June 2001

How much would you expect gas mileage to deteriorate by switching from a 30 weight oil to 40 weight? From 30 weight to 50 weight?

Hypothetically starting at 20 MPG with 30 weight ...

adc100 · June 2001

30 to 40- .5 to 1.0 mpg.
30 to 50- 1.5 to 2.0 mpg.

mrdetailer · June 2001

I have an older car and recently switched to a synthetic 5W-50. It has a small 1.8 liter engine and works very hard. When I added the heavier oil I got about a 10% drop in mileage, but my oil consumption went from 3 quarts per 6,000 miles to just a pint.

According to general knowledge Synthetic has less friction. Generally they say in Oil a slippery subject part II that you can go up from 30 to 40 without any loss in mileage.

andre1969 · June 2001

...was always to throw straight 30W oil into it! But then the most modern car he ever owned was a 1985 Buick LeSabre, which still uses it to this day. I'm guessing it wouldn't be a good idea to dump 30W into my Intrepid!

I do remember some friends of his had a 1982 or so Bonneville G with a 231 that ran poorly. He changed the oil for them, putting in 30W, and it ran fine. When I bought an 11 year old '82 Cutlass Supreme with the 231, I put 30W in it, and it ran fine. But then one of my friends told me that I didn't need oil like that in a modern engine, and would get along fine with 10W-30, along with better economy. Weeell, about a month after I changed my oil, with 10W-30, it lost all oil pressure! Ended up changing the gears in the pump, which were all chewed up from engine fragments, and it ran. Not well, but it held oil pressure. Also needed a quart every 300 miles or so. I took it to my local garage, who said I should shoot it there and not put any more money into it.

I'm wondering now if that car was on its last legs to begin with, and putting the straight-30 oil in just prolonged the inevitable. I wonder how long it would've lasted if I'd stayed with the 30. Well, I guess when you pay $800.00 for a car, you can't expect it to be perfect!

brorjace · June 2001

Andre, once a car NEEDS straight 30 (orn 40, 50, etc ...), yes, it's usually on its last legs. STP oil treatment can also help in these cases. The thicker oil makes a thicker film that hides the noises that worn parts (like bearings) make (knocks, taps, etc ...). Essentially, it fills in the gap where metal used to be.

I'd be afraid to use a thick, straight-weight oil in a brand new motor. There are tight, tight tolerances that the thicker oil may not make it into in sufficient properties for effective lubrication. Use the thinner stuff (5W30, 10W30) until your motor shows some evidence of wear and then gradually move up to the thicker stuff like 1040, 15W40, etc ...

acd100, I'd never express even a mileage gain guess in terms of miles per gallon. Whatever you think of the issue (and I DO think that synthetic oil which flows easier and has a lower coefficient of friction provides better mileage) you should probably post guesstimates in gains in percents.

For example, if I have a Honda Civic that normally gets 40mpg and my brother drives a Ford Expedition that gets around 12mpg and we both switch to synthetic, we could both gain about 2-3% more mileage. For me that would be 1.5-2.0 mpg but for him, it would mean an increase around 0.5 mpg. See how that process works out to more realistic numbers?

--- Bror Jace

zr2rando · June 2001

And haven't had any change in gas mileage. Truck has about 170k and I use 10w40 with STP (started the stp at about 150k). The stp definitely makes the engine quieter like brorjace said above I'm sure for the same reasons . I also think the stp makes the oil "sticky" , because when I crank it up it is also smoother at start than it used to be. Acts like it doesn't all run off the surfaces like it used too. I think normally thinner oil will help the manufactures get some better gas mileage stats "theoretically at least" but thicker oil gives better protection, maybe "theoretically" at a gas mileage cost....

adc100 · June 2001

I was looking at a middle of the road car (low twenties mpg). There are so many variables in this equation. Driving habits, or whether it's trip or city. These would yield different percentages for a given engine. For instance one person gets 18 mpg out of a 2L engine and another person gets that out of a 5L engine. Will the mpg savings on both be the same percentage? I doubt it. The guy with the bigger engine gets more benefit-without a doubt. The numbers I used to see in Industry for various equipment were 1% to 15% (probably bogus also). Some equipment we converted to syn yielded zero savings on motor amp printouts. (These were less than .1% inaccurate.) The reason of course is the percentage of the total energy lost in the bearings or areas where the oil has an opportunity to effect economy is low. Worm gears for instance are huge candidates for savings, and large Pump/motor combinations with plain or antifriction bearings of say 500 Hp yield little if any savings.

As a result, the management talking heads nixed the conversion on the basis of no percieved energy savings. At least I got a performance award for initiating the project. Works for me. He-he.

brorjace · June 2001

adc100, good points. I'm glad you put a lot of thought into your answer. It just didn't appear that way to me when I first read it. >;^)

You're right, there are plenty of variables but you hope the pwerson asking the question understands that going into the discussion. Like my car LOVES jihj temperatures. Once it got into the 80s and 90s around here (Saratoga, NY) my car went from 39-42mpg (fall/winter/spring) to 43-45 mpg.

zr2rando, STP is viscosity improver like that used in most multi-viscosity oils. The stuff works (as you've found out) but you have to watch it under severe duty service (extremely high temps and RPMs). Those conditions can break the long-chain STP molecules down and you are left with varnish and goo inside your motor. So, either keep the conditions to the point where the STP won't be broken down or change your motor oil & filter fairly often ... like 5,000 miles MAX.

--- Bror Jace

zr2rando · July 2001

Bror Jace:
I filled a mayo jar with used 10w30 and another jar with used 10w30with STP and compared them to each other (visual comparison, not very scientific..)
Out in the garage (temp about 85F I live in NC) the stp oil looks a little smoother, when you shake it up and watch it drip off the top it drips in longer strings from the lid...
I put both jars in the freezer and expected the stp oil to really thicken up more than the 10w30nonstp but they both looked just as thick.
I change oil every 3k miles so hopefully I'm not pushing any limits too badly
The stp bottle says it contains "olefin copolymer"
I think I've seen that listed as a componant in synthetic oil too (not positive though) any comments on that ? is that the Viscosity Improver your talking about?

oldharry · July 2001

a decrease in power/rpm with a four cylinder motor cycle when I tried heavier oil. I did not leave it in long enough to talk about gas mileage. In modern cars if you use too thick an oil, it may not properly lubricate all the bearings soon enough. With the tight clearances now found, cold heavy oil may not flow though the engine properly. This can be grounds for denial of warranty.

Harry

rayfbaird · July 2001

If you move up to a heavier weight it will have less effect.

I've found in the heat that air conditioning has far more influence than an increase in oil weight.

Whether to move up depends on a number of things. How old the engine is, Manufacturer's recommendations, and whether synthetic is used. Many in the oil sites have said you can go up one grade (i.e. from 30-40) with synth without much influence.

tronsr1 · July 2001

I have a new Ford Escape...manual calls for 5-20W and I believe some of the newer Hondas also call for same. It is my understanding that the reason for this change to 5-20W is for an "increase" in mpg. This helps manufacturers get a better CAFE rating.Most major oil companies are finally coming out with the 5-20. Motorcraft, Exxon Superflo and some others are now available at autoparts stores, including Walmart.

adc100 · July 2001

What you say may be true, but the deal with syn is that you can go to one grade thinner and still have the same lubricating. There are exceptions of course. This also holds for industrial applications (pumps/motors)with ISO grade oils.

rayfbaird · July 2001

I spoke with Quaker State Technical Dept. They have a 5W-20 and it had to add Synthetic to meet wear standards. I'll bet most manufacturers are doing the same thing.

QS 5W-20 already meets the new SL standard. the 5W-30 should soon, but I wouldn't put in 5W-30 until it qualifies for the highergrade.

Synthetic already meets the higher standards, so a 5W-20 Syn might be the best. At least during the warranty period. After that, the SL should be out, and you can move up to a 5W-30.

brorjace · July 2001

I've seen Pennzoil 5W20 at K-mart. It would not surprise me that ALL manufacturers use either some synthetic or severely hydrocracked oils in their 5W20 oils just to make them protect well enough to pass current standards. 5W20 is frighteningly thin.

Speaking of standards, are we SURE that SL will be better than SJ? We know one of the things that changed when they moved to SJ is that the amount of ZDDP (zinc phosphate), an important anti-wear additive, was significantly DECREASED. SH oils may actually be BETTER for your car in the long run. Look at some of the racing formulas ... many are SH rated.

--- Bror Jace

mrdetailer · July 2001

I switched to a 5W-50 Synthetic 8 months ago in my Subaru. I compared 2 cars, My Mazda runs Conventional. Generally the Subaru got lower mileage because of the 4 wheel drive. Last Saturday I got to compare. Both had mixed city/highway driving. My wife took the same 100 mile trip in each car. Air conditioners used extensively.

Results:
Subaru 27 MPG
Mazda 26 MPG

I'm not going to worry about the heavier weight.

brorjace · July 2001

Mr. Detailer,
I wouldn't use a wide-spread motor oil. The polymeric goo that is used in them is an oil's weakest link. This stuff is the first to break down in the oil (after only a couple thousand miles) and when it goes, your protection is compromised and the leftover, broken goo can sludge up your motor.

If you use it, don't EVER leave it in for any more than 5,000 miles.

--- Bror Jace

tronsr1 · July 2001

Thanks for the info.Keep on posting and keeping us updated.

rayfbaird · July 2001

Valvoline now has a 5W-20 that now meets the SL standard.

mrdetailer · July 2001

I called Quaker State technical support line. The PAO base is 10W-30 without any additives at all. Being multigrade in nature, it actually takes fewer additives than dino.

They also said this about their mileage.

A 50 Weight synthetic will take about 1-2% reduction in gas mileage over a 30 weight oil.

Conventional oil reduces mileage performance by 1-2%.

So roughly, a 50 weight synthetic has roughly the same performance as a 30 weight conventional.

adc100 · July 2001

If that statement is true (which I doubt), I would be reluctant to use the QT. The upside would be that an ester would not be required to make an additive soluble with the PAO and there would be no multi viscosity additive required.

The down side is that there is no anti-wear package and TBN additive. Mobil ran lots of tests against competitors' syns and many did not meet TBN and wear requirements after tripple and quadruple Sequence IIIE, VE tests (API). This would imply that QS could not be used for extended drain intervals. I have found that the guys that answer the 1-800 calls for the oil companies may just be talking heads. Others on this board may be able to shed more light here.

mrdetailer · July 2001

And do you have a site where this Mobile one Testing is listed?

adc100 · July 2001

SAE

http://www.sae.org/servlets/index

The report is Technical Papers Series- 951026

Pretty lengthy report-worth the 10 or 20 bucks I paid for it.

Oil Weight vs. MPG

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