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#62296 - 01/07/04 02:21 AM Ok to mix 10W30 and 5W50 US Castrol Syn?
bob555 Offline

Registered: 01/04/04
Posts: 74
Loc: Midwest
Before I knew better, I bought a 5qt jug of Syntec 10W30, which I finally decided to use up last oil change on my winter beater. Problem is that I happened to get a crappy SuperTech Filter with a Anti-drainback valve that didn't work (I got to watch the gauge stay on the peg way too long on each cold start). Anyway, so at 2,000 miles into what I planned as a 4-5,000 mile interval, I swapped filters, and accidentally bout 5W50 to top up with. Are the additives in different wieghts of US Syntec compatible? Can I just assume that I can safely run this mixture for another 2,000 miles?

#62297 - 01/07/04 02:34 AM Re: Ok to mix 10W30 and 5W50 US Castrol Syn?
csandste Offline

Registered: 05/31/02
Posts: 882
Loc: St. Peters, Missouri
If you can easily take the oil back and swap for the other weight you might want to do so, if you've already mixed and it's in your engine I wouldn't worry, nor would I dump the oil early.

#62298 - 01/07/04 03:53 AM Re: Ok to mix 10W30 and 5W50 US Castrol Syn?
Dr. T Offline

Registered: 05/28/02
Posts: 2480
No problems whatsoever....even if you topped up with any other brand, let alone the same can even stick with the same OCI that you normally do.

#62299 - 01/07/04 04:28 AM Re: Ok to mix 10W30 and 5W50 US Castrol Syn?
csandste Offline

Registered: 05/31/02
Posts: 882
Loc: St. Peters, Missouri
To expand on Dr. T. I think additive clash is overstated--most engines don't fail readily unless they lose coolant or oil and overheat. I've owned more than 25 cars in my life and only two or three of them have died because of anything related to the engine. Most of the time other repairs make them too expensive to maintain. Who wants to put $300 into suspension work on a '93 Topaz with 150K?

#62300 - 01/07/04 05:25 AM Re: Ok to mix 10W30 and 5W50 US Castrol Syn?
Patman Offline

Registered: 05/27/02
Posts: 19471
Loc: Oakville, Ontario
Additive clash really only comes into play if you were going to be doing a long interval, since the mixing of two radically different types of oils might cause the oil to oxidize faster and/or the TBN would drop faster.

I personally don't think this would be much of an issue unless you tried to go over 10,000 miles on it, however it would probably still be a good idea to go no more than 5k to be on the safe side.

#62301 - 01/07/04 08:22 AM Re: Ok to mix 10W30 and 5W50 US Castrol Syn?
TSoA Offline

Registered: 08/12/03
Posts: 425
Stephen Miller of Shell Oil answers this nicely...

"All grades of Shell oils, synthetic and conventional, can be mixed with no compatibility issues," he says. Miller is a Shell research engineer.

Shell has also tested compatibility of products with our four major motor oil competitors. While we can't always be sure of what changes they may be making to their formulations, we currently are not aware of any compatibility issues.

"It is certainly true that there are additive technologies and synthetics combinations available that could cause incompatibilities," Miller says. "But Shell does not use them, and I know that if that ever changed, we would be sure to keep everyone very well informed."

At the same time, it's important to understand just what constitutes incompatibility.

"Incompatibility of engine oils refers to observations such as additive dropout, or changes in fundamental oil performance that's not what one might expect from mixing two different oils together," Miller says. "But naturally, if you mix oils with different characteristics, you're actually blending those characteristics - you've changed some of the features of the oil."

For instance, if you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product should work as expected.

"Using our Shell products as an example, if a car specifies a 5W30 oil as the preferred viscosity grade, then FormulaShell 100% Synthetic 5W30 provides the best performance we have," Miller says.

"And if you find you need to top up with another Formula Shell there will be no compatibility issues, but the mixed product's performance characteristics will not be the same as if only FormulaShell 100% Synthetic had been used.

"But there's absolutely no danger about incompatibility resulting from mixing Shell engine oils," Miller says. "We've tested all of our grades and brands, and we haven't observed any problems."