Topping off 5w30 Syn with 0w30 Syn between OCI?

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Well I change my Pathfinders oil with Castrol Edge EP 5W30 Syn with K&N oil filter. I recently came across a bunch of 0W30 Mobil 1 full syn for $1 each quart at AutoZone. I am wondering if I'll be fine topping off with 0W30 will cause any issues since its a different weight and brand. Thanks in advance!
 
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it's a mixed bag as to whether or not that's bad. Will it blow up your car? No. Will you damage your engine? Nah. Could it alter the overall performance of the 2 oils, possibly resulting in poorer performance. Maybe. I've done it for years in the caliber, but don't extend the OCI past what the OLM says to.
 
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0w and 5w mix should have little bearing unless you're really dependent on that 0w rating in a very cold, winter climate. In summer, it probably makes zero difference. It's not the viscosity difference or brand I'd be concerned about. But, the fact that the Castrol is a high Calcium, no magnesium oil, while M1 is low Calcium, high magnesium. You'll end up with medium Ca and medium Magnesium....something that probably no one sells or certifies. That said, probably not much of an issue to top off as you suggested. You'll be slightly diluting the Ca of your Castrol, and adding magnesium which wasn't there to begin with. A better choice for makeup oil to Castrol 20/30 grades (if you have no Castrol) is PP/QSUD/FSS/MSS which are quite similar. Other than Valvoline and M1 20/30 grades, most other oils out there have similar chemical add packages to the current Castrol.
 
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Originally Posted By: 69GTX
the fact that the Castrol is a high Calcium, no magnesium oil, while M1 is low Calcium, high magnesium. You'll end up with medium Ca and medium Magnesium....something that probably no one sells or certifies.
They serve essentially the same function. I wouldn't have ANY problems mixing the two. Both good oils - - and GOOD SCORE on $1 a bottle!!
 
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Originally Posted By: Linctex
They serve essentially the same function.
But do they work linearly and cumulatively ? i.e. does 0.75xCa and 0.25xMg = 1 in terms of function ? If you have evidence of that then please post, enquiring minds would like to know, or is it just a guess ? OP, you'll be fine...
 
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Originally Posted By: SilverFusion2010
It's not ideal, but adding some extra oil of a different brand is better than running with a low oil level
Of course this is the best option. But if you have a choice, add the most compatible oil available to you. If this wasn't the case there'd be no reason to buy 5-6 quarts of the same oil. A witches brew of various brands would likely be medium/low Ca, low to med Mg, and low to medium Na. There's no such brew out there made by any vendor that I can see. If it were the ideal mix, someone would surely be making it...or had discovered it. So either it's not the best mix, or technology has not gotten there yet to prove how great it is....a little bit of every chemical. I suspect that could induce too many chemicals competing for the same goals and interfering with each other. Chemistry is a lot easier to map with just 2 reactants rather than 3.
 
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