can i mix 5w30 with 10w30?

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i have gm 5w30 oils in the engine.i was thinking to put gm 10w30 in it,about 1/2,1 quart.is it ok,or should i only add 5w30
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I've even mixed straight 30 weight in with 10w30 and no apparent problems. Oh, I've mixed worse things than that too.
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how about mixing 76 super synthetic blend about 1/2 or 1 quart to gm dino 10w30 4 quarts?
 
The net effect of adding 1/2 or 1 gt of anything
in your 4.5 gt really doesn't change it much
as you can check in the shell viscosity mixer as its only changing it by 1/5 anyway. I do use add oil to aim the grade to where I want to try.

On my daughter's oil consuming tercel, I add 1 qt of 20-50 Castrol HM to see if thicker helps stop her consumption and rear main leak. On mine I add some moly laden VSOT or 10-40 to see if moly or thicker helps. Its like a low cost preview if you are going to change in 1k miles anyway.

On yours, you will change soon if you stay with 3K OIC so adding a blend with maybe 20% syn is like adding 5% syn. If you want to experiment and see how a blend works with GM dino I'd add all syn so at least you get a blend for the entire 4.5 gts. Costwise I'd add any dino since you'll drop it in a month.
 
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