Castrol 0w40 specs

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Nov 15, 2002
I e-mailed Castrol in Germany last night and requested the data sheets for the Formula RS 0w40 and 10w60, as well as the TWS 10w60. The sheets they sent me are all in German, but here's the relevant data on the 0w40. I'll post the 10w60 specs later. (The Formula RS 10w60 and the TWS Motorsport 10w60 don't appear to be the same oils.)

Castrol Formula RS 0w40

Pour Point: -61*C
Falshpoint: 236*C
Viscosity @ 40*C: 75.2
Viscosity @ 100*C: 12.9
Cold Cranking Vis @ -30*C: 3250
Viscosity Index: 173
TBN: 11.1

A couple of things jump out at me. First, this oil is formulated on the low, low side of the 40wt range. At 12.9, it's barely a 40wt. Second, the VI is actually lower than the SLX 0w30. This tells me the viscosity spread is accomplished almost exclusively with the high VI base oil blend.
I like the low viscosity 40wt, I think a lot of cars would show good wear numbers with that viscosity.

Although it really doesn't look to be that far off the 0w30 Syntec we now have here. At 12.1 cst it's not a massive difference in thickness. These two oils are probably very similar in their creation.

Ya gotta love that pour point though. It's simply incredible, and shows that it must be using a pretty darn good base oil.
I find many of the Euro 0w-40's are formulated this way ...they are just thick enought to meet the 3.5 Cp minimum for HT/HS viscosity required for the ACEA A3/B4 rating. The 5w-40 syntec used in the VW TDI diesel comes in @ 12.8 Cst @ 100C - in other words if it was 2% thinner, it would technically be a 5w-30. The Mobil 1, 0w-40 comes in @ 3.6 Cp in the high temp/high shear test as just another example.

I think the idea is to meet the spec but to optimize fuel efficiency at the same time.

The beauty of this oil is, I truly believe a lot of cars on the road today would show better engine wear numbers with a higher 30 to low 40wt oil. An engine such as my LT1 or the LS1 engines too, would benefit from it. Anything that has a looser clearance like this, and engines with a bit higher miles on them too. But a lot of the 10w40s out there end up being a bit thicker, in the 14 to 15cst range, so they end up just hurting the fuel economy while not really protecting better.

The trend with a lot of oils, such as Mobil 1, seems to be to formulate it at the lower end of the 30wt scale. This works well for new tighter clearance engines, but I think these oils in higher mileage and looser clearance engines end up sacrificing engine wear for fuel economy.

This is why I want to do my viscosity test this summer, mixing Schaeffer 15w40 with 10w30 in a 50/50 ratio, to see if engine wear is lower in my car with an oil that is in the higher 30 to lower 40wt range (12-13cst specifically) Who knows, maybe I'm way off base, but I have a strong gut feeling I'm correct. A hard driven car like mine just needs a little bit more "cushion" in there. And with these slightly thicker oils, the HTHS goes up too, which is definitely a good thing.

I think Amsoil agrees with me on this one too, since their 5w30 and 10w30 are formulated at the top end of the 30wt. I know this is partially done in order to keep it in grade for longer intervals, but I also believe they are thinking in terms of engine wear even for those that do shorter drain intervals too.

[ February 19, 2003, 11:19 AM: Message edited by: Patman ]

I think you are 100% correct ...most xw-30 grades are formulated for maximum fuel efficiency, so they are down in the 9.5-10.5 Centistoke range. I think it makes more sense to make them thick enough to meet the ACEA "A3/B4" specs for high temp/high shear viscosity. Almost every multrgrade oil I've seen thins out a bit in service, so this gives you some margin to work with.

Of course, you can always go the 0w-40/5w-40/10w-40 route, but few cars call for those grades, outside the German hardware. So if you car is under warranty and you want to follow the letter of the law, you're kind of stuck.


I'm under warranty but I *can* use 10W-40 in one of my cars (or even 10W-50, for that matter, as well as 5W-30 and 10W-30). Trouble is, finding an API SL synthetic in that weight... Am I missing info on an oil I should know about?


The API SL/GF-3 spec has certain fuel efficiency requirements, so you normally don't see 10w-40 grades meeting this. The two best 10w-40 oils I know of are the Amsoil 10w-40 and the Redline 10w-40. If you want a licensed product, the Mobil Delvac 1, 5w-40 is a very good oil as well. I like the additive chemistry of Delvac 1 over the Mobil 1, 0w-40 for a severe service application. You'll find it stays in grade better ....

Thanks, you've confirmed my suspicions. I want to run Delvac1 5W-40 or M1 0W-40 so badly, but alas, those specific grades are not listed in my manual...
rALLYFAN DON'T GET HUNG UP ON WHATS IN YOUR MANUAL. oops. I have an older (14yrs) turbo vehicle that only has 10W40 20W50 etc in the owners manual but I've used 10W30, 0W40 M1, several 5W40's icluding Delvac 1 and BP Visco 5000, 10W40 Catrol Magnatec (temporary insanity), and every 20W50 you can name and even 15W60 and 25W70. 120,000 miles and like new (thanks Auto-Rx). Currently using a M1 0W40/Delvac 1 blend. Redline 5W40 would be my top pick though if cost no object.
Oh yes, I agree: any of the better oils mentioned will provide excellent protection and longevity. I'm not nearly as hung up with my manual as the manufacturer is though - I simply don't want to get into questions/trouble with the dealer if something seemingly irrelevant goes wrong and oops, I've used a non-spec'ed oil. I'd love to use M1 15W-50 or 0W-40 or Delvac 1 but I won't while under warranty.
Mobil states on there website that the 0w-40 can be used in any car that calls for a 30wt. oil as well. The visc. on M1 0w-40 is 186. It would be nice if Mobil made a 10w-40.

[ February 21, 2003, 09:32 PM: Message edited by: buster ]

Originally posted by buster:
Mobil states on there website that the 0w-40 can be used in any car that calls for a 30wt. oil as well. The visc. on M1 0w-40 is 186. It would be nice if Mobil made a 10w-40.

Actually, what the data sheet says is that it can be used where a 0w30, 5w30, and 5w40 viscosity is recommended. I find it interesting that 10w30 is absent from this list.

And it's the viscosity index that's 185 (not 186).
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