Noticeable differences with Exxon/Mobil vs. Shell & others

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Would like some feedback on this as it seems weird to me.

On two different cars, over a span of 8 years, I've noticed a trend: If I run Exxon/Mobil almost exclusively for weeks/months, cold cranking takes noticeably longer (like 50-100% longer) and the engine runs a bit rougher on a cold start. With Shell primarily, supplemented by other brands (including Exxon/Mobil), cold cranking and cold idling are normal. 93 octane in all cases, which is what the cars called for.

The primarily-Shell plan was what I originally ran on my previous car. Then I switched to Exxon/Mobil almost exclusively for convenience. The cold start differences started several weeks later and persisted for several months. Then I switched back to the original plan and everything was fine after a few weeks.

The car I have now didn't seem like it'd be as picky, so when I got it I thought nothing of primarily running Exxon/Mobil for convenience. Over time, I started to notice the same cold start sluggishness. It persisted through battery, starter, spark plug, and emissions component replacements (done largely for other reasons). Then I started using Shell when I could and minimizing Exxon/Mobil, and again, everything was fine after a few weeks.

I don't remember anything else about either car feeling any different with different fuels. Hot starts, throttle response, power, warm idle quality, fuel economy, etc. -- it all seemed about the same no matter what fuel I ran. The only differences were in cold start cranking and idling.

Also, I never have cold start issues from an Exxon/Mobil tank here or there. Again, it only ever happened when I ran Exxon/Mobil almost exclusively for weeks to months.

Anyone else notice anything like this? It's exactly the kind of thing I'd be tempted to blow off as likely coincidence or superstition if I didn't experience it.
 
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Would I blame different brands of fuel for this issue? No. But it could have something to do with particular fuel stations. More likely I’d say its your vehicle with the issue, not the gas. Does your vehicle have a engine performance problem rearing it’s ugly head that you don’t know about. I ask because one of my vehicles does something very similar, certain brands of gasoline cause slow, hard cold starts. Switching brands resolves the issue within the first tank of switching brands. I put my code read on it one day and i discovered an intermittent DTC for the downstream o2 sensor for low voltage. It never caused a check engine light fyi.

It’s been happening for 5 going on 6 years now and I have yet to replace the downstream o2 sensor. It’s a spare vehicle so I’m not worried about replacing it until it gets real bad.
 
I've noticed a bigger difference between stations than it being necessarily Mobil vs. Shell vs. BP
 
Would I blame different brands of fuel for this issue? No. But it could have something to do with particular fuel stations. More likely I’d say its your vehicle with the issue, not the gas. Does your vehicle have a engine performance problem rearing it’s ugly head that you don’t know about. I ask because one of my vehicles does something very similar, certain brands of gasoline cause slow, hard cold starts. Switching brands resolves the issue within the first tank of switching brands. I put my code read on it one day and i discovered an intermittent DTC for the downstream o2 sensor for low voltage. It never caused a check engine light fyi.

It’s been happening for 5 going on 6 years now and I have yet to replace the downstream o2 sensor. It’s a spare vehicle so I’m not worried about replacing it until it gets real bad.
Good points. But again, I noticed this pattern twice, on two different cars, and it persisted through various forms of maintenance.

It also wasn't on or off within one tank. The cold start sluggishness took weeks of nearly-exclusive Exxon/Mobil use to develop, and only went away after several tanks from other brands.

These days, cold starts are nice and enthusiastic even on a tank of Exxon/Mobil (same stations as when I had the sluggishness), as long as I use other brands more frequently. It's better now than it was before, and the car sure as heck hasn't gotten any younger in that time.
 
Shell and XOM gasolines are rumored to use the same additives in their fuel except for tracer molecules. They both use their joint venture company, Infineum, for their additive needs. I’ve always considered Shell and XOM gasolines to
be the same, and random news/media outlet stories throughout the years have always confirmed that Shell, XOM gasolines always have the highest amount of detergent additive compared to other brands. However, that may have changed in recent years as more fuel retailers joined the TopTier program.

With that being said, it’s not unheard to have a vehicle that performs poorly on a particular brand of gasoline. It’s hard to pinpoint as to why, but it usually due to engine condition ie, carbon buildup, imbalance of compression in one of the cylinders. Or it could be in your head. It would
be interesting to see fuel trim data and o2 sensor voltage.
 
I have not detected any difference between Exxon/Mobil and Shell. I experience no difference between any fuels in my Toyota whatsoever. But my Subaru occasionally experiences extended crank times using BP or Chevron. So maybe your car doesn’t like Exxon for some reason; it can happen.
 
I use Exxon fuel exclusively (87) and have for about 5 years. No odd starting characteristics.
The car can sit for 2 weeks and it fires up before I can get my hand off of the key.
 
I don't really get why any fuel would be any different except maybe long term if the additive level isn't ideal for maintaining engine cleanliness. It's all commodity fuel with some extreme exceptions (like places with 100+ octane racing unleaded).

I will buy from a local Exxon station but I'm not wedded to it. Costco is consistently the cheapest in my area, and they don't even have a cash price to get it. XOM doesn't have a single refinery making California reformulated gas.

Granted - years ago a high school friend was convinced that Exxon and Arco gas was trash, while Chevron and Shell were the best, with maybe Mobil in there with those two. Didn't really have anything to go by other than marketing. When I pointed out that the refineries shut down (and we lived near them) such that they obviously have to do exchanges, he shifted to the quality of the individual gas station. He was also convinced that things made in Germany or Japan must be inherently better. Also wanted Z rated tires for his econobox.
 
This might be a car-specific thing -- not just make and model but individual builds. My previous car was a Mazda RX-8; my current car is an '08 BMW M3. Both have low-volume, very high RPM, largely hand-assembled engines that are unrelated to other models (especially the RX-8). Plenty of scope for both cars to have been weird in some way.

TBH, even if what I experienced was 100% real, I don't think it'd point to any real "problem" with Exxon/Mobil. Each additive package is formulated to balance performance, lubricity, cleaning, cold starting, etc., right? Maybe Exxon/Mobil does it in a way that sacrifices cold starting for some other attribute I can't feel or measure.

#ShrugLife
 
What you describe is really interesting given that gasoline from all of those stations comes from the same pipeline.
it is literally the same stuff.
The base fuel, yes. Not necessarily the additives.

But you see drastic differences in performance.
Hmmm....
Nope. Like I said, it was only on cold starts, and only when I used one brand almost exclusively for weeks/months, and I didn't notice any other differences (power, mpg, hot starts, etc.).
 
Would like some feedback on this as it seems weird to me.

On two different cars, over a span of 8 years, I've noticed a trend: If I run Exxon/Mobil almost exclusively for weeks/months, cold cranking takes noticeably longer (like 50-100% longer) and the engine runs a bit rougher on a cold start. With Shell primarily, supplemented by other brands (including Exxon/Mobil), cold cranking and cold idling are normal. 93 octane in all cases, which is what the cars called for.

The primarily-Shell plan was what I originally ran on my previous car. Then I switched to Exxon/Mobil almost exclusively for convenience. The cold start differences started several weeks later and persisted for several months. Then I switched back to the original plan and everything was fine after a few weeks.

The car I have now didn't seem like it'd be as picky, so when I got it I thought nothing of primarily running Exxon/Mobil for convenience. Over time, I started to notice the same cold start sluggishness. It persisted through battery, starter, spark plug, and emissions component replacements (done largely for other reasons). Then I started using Shell when I could and minimizing Exxon/Mobil, and again, everything was fine after a few weeks.

I don't remember anything else about either car feeling any different with different fuels. Hot starts, throttle response, power, warm idle quality, fuel economy, etc. -- it all seemed about the same no matter what fuel I ran. The only differences were in cold start cranking and idling.

Also, I never have cold start issues from an Exxon/Mobil tank here or there. Again, it only ever happened when I ran Exxon/Mobil almost exclusively for weeks to months.

Anyone else notice anything like this? It's exactly the kind of thing I'd be tempted to blow off as likely coincidence or superstition if I didn't experience it.
I noticed similar pattern even though the other way around... in my area Shell use extra friction modifier(FM) except for the highest grade of VPower, this FM I believe create longer startup compared to Exxon/Mobil or Shell VPower. I have observed this on 2 different cars, 1 with PFI and with GDI. However, my favourite cleaning is still Chevron or Shell VPower, it is just the price is not as competitive as others, so I guess price does matter. Now I am testing the local additive that seems better result than going higher octane and it is still cheaper, let see what happen after a few tanks though
 
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