Here in south OC, I have been fearlessly putting Costco gas in my cars. I shop there and nothing emits hints of low quality and I appt that concept to their gas as well.
I did notice last week that an annoying ping sounding sound from the engine stopped with v power from shell. I have shown this to dealer and they stated normal, but I was never convinced.
Now I suspect less than advertised octane rating for Costco gas and am afraid of going there for the 63 at least.

This was an issue as well with my old K1 C63. I now have a P31.

Has anyone else noticed this round these parts?!

I know guys that have filled up with Costco gas with no problems.
I haven't used they're gas so don't really know. Try getting some fuel system cleaner from Chevron and use it at your next refueling.
Been using Chevron, Mobil-1, 76, and Shell with no problems.

Stay with the top tier gas companies. The octane rating may be the same with Costco gas, but it's the additive package that makes the other brands cost more. You can always add a bottle of Techron every six months if you want to keep using Costco gas.

i don't get gas there but I sure as hell stand next to the kosher weenie lady giving out samples and just keep eating until she says, "young man, now that's enough now..."

Then I move down to the Fiber One bar samples and have about the equivalent of 6 whole bars...

I'm good to go then to wait in line for an hour for my 75 pack of tuna and 42 pack of frozen taquitos...

Also check the ethanol % content. Most of Shell. Mobil and Sunoco are 0%. I don't know about MBZ, but BMW had (or still having) problem with high % content of ethanol. Problem with the fuel pump. It makes sense since ethanol attracts water vapor that eventually mixed in the tank.

I guess that with all the money you save on food you don't have to buy your gas at costco

The last time I asked a SoCal Costco gas delivery driver where he picked up his premium grade he said "This time at Exxon."

That's all I needed to hear, Costco gas = FAIL.

From the Exxon/Mobil website:

Exxon and Mobil branded gasolines meet TOP TIER Detergent Gasoline standards. The gasolines we sell come in three octane levels and have been developed on a molecular level to help clean up intake valves and help keep vital engine parts like fuel injectors clean. Our diesel fuels are blended to provide good ignition quality and are held to high quality standards for our customers.

Zephyr,
I'm sure I've seen you there. I go for free lunch once a week.

Here in Ohio our Costco gas is distributed by Shell. It varies from region to region. I would ask the guy working the pumps if he know who the distributor is.

i been using Costco for the past 3 mercs and 1 bmw, no issues at all. (knock on wood)
like the op stated, nothing there emits hints of low quality.

also, if something do go wrong, i imagine costco will be more willing to be liable than some mom/pop gas station regardless of shell/exxon/etc..

The octane rating may be the same with Costco gas

Been using Costco since 99 on my previous cars and two current mercedes. I also every 3 months add techron or bg-44 to my cars. When i pass the 7/11 Citgo and see high end luxury cars fill up there, i'm like no mang dont do it.

Too Funny Duane....I frequent the one in Carlsbad off Palomar Airport, then the one in Carmel Mountain, then by the time I get to the one in San Marcos I am all full...

I used to run costco gas but noted about a 50 mile reduction in my tank range so the 3% savings was not worth the loss in fuel economy. Do your own test, run costco for a few tanks and then run chevron or shell.

I work for an integrated oil & gas company in Canada. Let me provide some insight on how the industry works when it comes to supplying Costco and other similar retailers with gasoline.

1) Most second-tier branded gasoline stations are supplied by one of the majors.
2) Octane is octane, so if Costco says the gas is XX Octane, it will be - their supplier controls that. Barring any blatantly poor operating practices at the specific station, you generally don't need to worry about 'bad gas'.
3)Each of the majors has their own proprietary additive package that exceeds industry minimum quality specs (i.e. Top Tier). They reserve this exclusively service stations of their own brand.
4)The additive is mixed with the base gasoline through injection at the time it is loaded on the truck headed for the service station.
5)The majors provide only the industry minimum quality additive to gasoline headed for 'other branded' service stations. So, even if Costco gas came from Exxon or Shell, it is not of the same quality that would be found at an Exxon or Shell station. The Costco gas would not meet Top Tier specs.
6)The difference between top tier gasoline and others is essentially the amount of detergent in the gasoline. Top Tier gas will result in less build-up of deposits on the valves, etc. If you are leasing your car for a couple years, you might not care about this. If you plan on keeping it for some time this should be an important factor in your selection of gasoline.

I only questioned octane because I hear sounds similar to ping. If that is what it is, it is not good for the engiene, even if it is a leased car. I also know that some imports recommend higher octanes than CA stations sell (now 91, used to be 92). I specifically remember one of my older cars (I think an E46 M3) recommended 95 octane!

I know that high compression performance tuned engienes are sensitive to potential variables in the fuel and even though I have not had any problems with costco brand gas in the past, I am now staying away from it and am using only "top tier" to see if I see a difference.

Thanks for all the info!

Looney 100 is exactly right. I have worked in retail petroleum for 20 years and the only item I would add is:
Major brands like BP, Shell, etc. additive packages vary by gasoline grade. So for premium grade, the amount,and may be type of additive, are different than for regular.

This is the case with non top tier gasoline. The company I work for used to put more additive in Premium than we did Regular, but Top Tier requirements forced us to increase additive levels in all grades.

The M156 pings on CA 91 octane gas. You can't get around it. I find that Shell 91 octane is pretty good for what it is. Chevron is also good, but more inconsistent from station-to-station than Shell.

Beyond pinging, the car just doesn't run as well as it does when higher octane is used. If you want to try an experiment, mix in some higher octane, oxygenated fuel. Then you will understand what the C63 really is meant to feel like.

East coast C63 owners experience a different car than West coast owners. When higher octane is used, the car idles more smoothly, it feels more sharp and throttle response is more crisp. And obviously, full timing can be advanced, so you can ultimately feel all of the hp that your engine can produce.

I think you have an excellent point. I think that "it is what it is" is what we have to deal with here in CA.

Have you had any experience with octane booster additives?

In Illinois and Indiana, 93/94 octane runneth over from every fountain of youth with the name Shell, BP, and Mobil....