FAQ - GC ( German Castrol )

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Jan 4, 2006
Sunnyvale, CA
I wrote this to answer the basic questions people have about GC. The forums are intimidating and finding information can be tedious.
I hope this helps anyone looking for basic info.

If you see any errors or have suggestions, please PM me.

Created: April 3, 2006
Last Edited: April 15, 2006

This FAQ was written for the U.S. audience. Not everything here applies if you live in another country.

Basic information:
  • What is GC?
    GC stands for German Castrol. It's officially named Castrol Syntec 0w-30.
    I'm not sure who coined this term.
  • Why is it called GC?
    It is the ONLY Castrol Syntec made in Germany.
  • What makes this stuff so good?
    With any oil, it may not work well with your car. However, the majority of us love it.
    • Produces great UOA results.
    • Engines run quieter and smoother. (OK, that may be an opinion.)
    • Comparable price with other synthetics. ~$5 qt
    • GC is a TRUE synthetic. Made from PAO/Esters, not Group III. Click here for more reading.
      • The other Castrol Syntecs are not true synthetics.
        Other examples of true synthetic oils are Mobil1, Amsoil, Redline, and Royal Purple to name a few!
    • GC is good enough to have its own forum! That's gotta mean something.
  • What's bad about this stuff?
    Not much.
    • You might have a hard time finding it depending on where you live.
    • Your car might not like it. You'll never know until you try.
      Keep trying oils until you find one you're happy with.
    • Addicting. You'll see what I mean...
  • Is it "thin"? Come on, 0w-30 has to be watery.
    NO! Please don't completely buy into that myth.
    • At cold temps, the 0w weight correlates to cold cranking viscosity.
      That means GC will still flow at cold temps. (Which is a good thing!)
    • At operating temps, the 30 weight is actually close to a 40 weight.
      GC is one of the thickest 30 weight oils around.
    I don't want to go into too much detail about this here. It can be very confusing.
    My point is, GC is not "thin" in most temperatures that we use our cars in.

    Click here for more details about the "Thin Oil Myth".
    The example uses Mobil1 5w-30 and 10w-30, but GC still applies.

    Click here for some viscosity comparisons.
    Click here for a chart about cold cranking viscosity.
  • Can I use this oil even if my car manufacturer recommends a different weight?
    Great question. Your best bet would be to search or start a new topic if it hasn't been discussed.
    It should be fine if your car needs 0w-30 (Duh), 5w-30, or 10w-30.
    Personally, I wouldn't use GC if my car called for non-30 weight.
  • Where can I buy this stuff?
    In the US, AutoZone is the only major store that carries GC. Most AutoZones carry it, but not all.
    In Canada, you can try your luck at AutoZone and Wally World.

More information you should know:
  • There are 3 types of GC.
    • U.S. - So it's not really GC, but it's U.S. made Castrol Syntec 0w-30.
    • Green - Yes, the oil is Green.
    • Gold - The most recent stuff.
  • What's the difference?
    • There's not much research done on the U.S. stuff, so just don't buy it.
    • Green vs Gold
      • No one is 100% sure which is better.
      • Green has proven itself to be great.
      • So far, Gold has been great too!
      • Gold is probably just as good as Green. (Maybe better?)
      • Click here for more reading.
    • Bottom Line
      • The U.S. stuff probably isn't as good as the German stuff.
      • Buy Green if you can find it. Otherwise go for the Gold!
  • How can I tell the difference?
    First, I'll teach you how to read the date codes.
    • Look at the bottom of the bottle, it'll say something like M05216xxxx
      • The 05 represents the year it was made. (2005 in this case)
      • The 216 represents the day it was made. (001 - 365)
    • U.S.
      • No need to look at the date code, it'll say "Made in the U.S.A." on the back.
      • It was last made in 2002, but many AutoZones still have some lying around.
    • Green GC
      • It'll say "Made in Germany" on the back.
      • Anything until and including M05010xxxx is Green.
      • This stuff came out in 2002.
    • Gold GC
      • It'll say "Made in Germany" on the back.
      • Anything after M05010xxxx is Gold.
      • This is the current GC.
    Click here for some photos of US and Green GC.
    The Gold GC bottles look similar or identical to the Green GC bottles.

Miscellaneous Questions:
  • Are you a chemical engineer or tribologist?
    Nope, just an enthusiast. I am an Electrical Engineer and I got A's in all my physics/chem classes.
    (I still don't know jack about those subjects.)
  • Why do people hunt for Green GC so frivolously?
    I don't know. There's something novel about losing gas/time/money/relationships for the Green.
  • What's the deal with the elves?
    Shhh, don't get involved. They'll pull you back in if you try to get out.

  • sxg6 for the GC photos.
  • AndyH for the cold viscosity table.
  • 427Z06 for the viscosity comparisons.
  • Everyone who gave me suggestions and caught my errors. If you think your name should be here, don't be shy and PM me!

Thanks for reading!

[ April 21, 2006, 10:06 PM: Message edited by: 59 Vetteman ]
Great synopsis! One thing: Yes, all Syntecs except 0w-30 are group III (i.e. derived from petroleum). Mobil 1, however, is PAO (group IV) and therefore a true synthetic. Thanks again.
Mobil 1 is a true synthetic (Grp IV,V), otherwise great job. By the way I was the originator of the viscosity chart. We should probably update it with current formulations.

Hope this becomes a sticky, eventually.
Aside from the M1 error, I'd modify the disclaimer to add that GC is also available in Canada, at AZ and Wally World, no less. I hope it does become a sticky.

p.s. "Yellow" GC isn't that hard to find...
I concur with 427 and ccs. Only thing I'd add is that in some places, there's still a substantial amount of yellow label out there, so I'd say it's actually easy to find and perhaps buy by accident if you're a novice to the whole GC craze. There's still a lot around the New Orleans area, for example. I'd add a stronger caution to look out for this stuff. Good job otherwise, thank you!
" That means GC is only thin at extremely cold temps. (Which is a good thing!)"

You may want to find another way to word the above sentence. Like other oils, GC is thicker as it gets colder. For the casual lay reader, you may want to change to something along the lines of:

"That means GC is still pumpable (or will still flow) at extremely cold temps.)
FD777, I think that you should track down whomever determined the lot number break - US group III to Green to Gold. Give them some credit. The availability of these numbers have prevented a lot of tops from being opened!
FD777, let me know which brands/grades you'd like to see in the viscosity chart and I'll make up a new one. We probably don't want to make it any wider than the current one and I would think we should stick with all "syns" for a more apples-to-apples comparison.

Originally posted by FD777:
[*]What's the deal with the elves?
Shhh, don't get involved. They'll pull you back in if you try to get out.
Thanks for reading!

Too late! The Elvish ones have already been introduced into the conversation. The deal with the Elves is this: in a thatched roof blending plant in der Schwarzwald (the Black Forest) Friedrich, Jürgen and Sascha led a team making some of das ausgezeichnetste Bier das bekannt ist zur Welt (the most excellent beer known to the world). The problem was they could never get it to the market at its peak of freshness because the truck kept breaking down (oil failure). SO, the diminutive ones blended and brewed their own oil. To make it obvious from other lesser formulae, they dyed it green with dye made from the green, green moss that grew in the little stream behind the blending plant. (It IS true that the original idea for the original color and fragrance comes from an incidental but not insidious induction of a batch of green Gummi Baren into the blending vat.) Now that GC has become the most exalted elixer ever experienced, they no longer need to dye it and so it carries its naturally brilliant glamorously golden glow............................and now you know The Rest of the Story.

PS For the record, I did coin the term GC but Dr T and Patman (and I believe G-Man II) did fan the original gloriously green and flamboyant flame with Dr T, I believe, actually making the original discovery.
Thanks for the input! I'd like to here some more feedback and make the changes in a few days.

I would also like to add in a credits section. Please let me know if anyone deserves credit (Such as 427Z06, AndyH, and the person who took the photos)

Yellow GC is hard to find where I live. (1/23 AutoZones had the U.S. stuff) I guess thats not true for the rest of the country

Didn't mean to be ignorant!

The Mobil1 info is a huge error on my part. I heard Mobil1 switching to Group III after they lost a lawsuit to Castrol.

I should never trust word of mouth info...

Mods, please kindly inform me how to edit my posts after the "editing time" has expired. Thanks!
You are seeing the same thing just worded differently.

The FAQ says, "Anything after M05010xxxx is Gold." To me that means M0502xxxx and on. Perhaps it should be reworded to read "M05020xxxx and on is Gold."

He clearly said that M0501 is the last of the green. "Anything until and including M05010xxxx is Green."

In the link that I provided, Bruce381 states that M05020XXX is GREEN and that M05028XXX is GOLD. Thus, no matter how clearly M0501 is stated as the last of the Green, I am left confused. If Bruce381 has a VOA of M05020XXX and it is GREEN, then something needs further clarification.

Did you read vad's post (4th one down in the aforementioned thread) about misplaced digits in the batch numbers? I think it was just a typo. That should be enough clarification.
Mobil1 and other Castrol Syntecs are not true synthetics.

That either baseless or negligent slander against ExxonMobil.

Mobil 1 SS has a PAO carrier oil. Domestic Castrol Sham-Tecs do not.

Are you an independent poster or employed by another party?


The Mobil1 info is a huge error on my part. I heard Mobil1 switching to Group III after they lost a lawsuit to Castrol.
I should never trust word of mouth info...

I'm an independent poster. As I stated above, it was an unintentional mistake.
Yes, I was negligent; I didn't do my research.

I have used Mobil1 in the past and pending my UOA results, I may continue to use it. I just put GC in my car, 4/8/06. If the results are worse, I will switch back to Mobil1. Simple as that.
So far, the GC is superb with my friend's Supra. (No pun intended.)
As for my car, WRX, GC seems marginally better.

I haven't updated the FAQ because I can't edit my posts. I'll change this ASAP (whenever I get moderator support)

[ April 14, 2006, 09:02 PM: Message edited by: FD777 ]

Originally posted by FD777:

- Why do people hunt for Green GC so frivolously?
I don't know. There's something novel about losing gas/time/money/relationships for the Green.

Because the aroma is so devine! I also prefer the fruity flavor...
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