Originally Posted By: cabanalane
As to the why question?
I have 3 cars that use 3 different weight of oil. I generally don't stock them at home. If I'm out or short, I'll use what I have.
I guess it's better to go 0-30 if I don't have enough 5-30? In reality, I may add 1/4 qt to top off. From what I have searched and read, it's ok. And have been doing that (when I'm out). But just wanted to know if that actually changes the weight of the oil.
Or there may be time when I got some 0-30 left, and if I'm doing an oil change, I may just use it up.
To make the math easier, I set up the question that way. No, I don't mix the oil on purpose.
So is it the "general" rule to go lighter? Meaning, I can add 0-30 to my 5-30 engine, but I should not add 10-30 to my 5-30 engine. Correct?
I'm a newbie - so I'm sure this has been discussed a million times here.
I believe you've got it. The second number, the 30, describes the viscosity while at operating temperature, so in essence, all your vehicles that want xW-30 have the same viscosity when up to temp.
The general rule, like you said, is that you can easily go lower in the winter rating. It might even be preferred in colder climates.
If all your vehicles want xW-30, I would suggest stocking and using 0W-30 in all of them and sleeping well at night.
Just noticed that you're in California. Wanted to add a note that in your climate, unless you're up on a mountain, even on your coldest days, the difference in viscosity between a 0W, 5W and 10W is relatively small (compared to the difference at 0F). I would still just use the 0W, but sleep even better at night.