BY TOM AND RAY MAGLIOZZI Modified: June 8, 2012 at 3:18 pm •  Published: June 11, 2012

Dear Tom and Ray:

I was buying oil for an oil change and was planning to use a coupon from the manufacturer. The auto-parts store was out of stock of the partial synthetic blend I use. So I decided to go with the full synthetic, because with the coupon, the price was almost the same. As I was checking out, the sales associate said (and I paraphrase): "Now, remember, once you use full synthetic oil, you have to always use full synthetic oil."

I looked at him and said, "Why is that?"

He replied, "Well -- that's what they say."

Personally, I think he was just trying to make sure I spend $10 a quart from now on, instead of half that for the synthetic blend. But who knows? Maybe he's right! Is he right? --Richard

TOM: I don't think so, Richard. We heard the same kinds of warnings when synthetic oil first came on the market -- not to mix it with conventional (dinosaur-based) oil, or something terrible would happen. But we never saw any hard evidence to back that up.

RAY: And then the manufacturers started mixing the two themselves! What do you think the "synthetic blend" you usually buy is? It's a blend of synthetic oil and conventional oil in the same container!

TOM: So if the manufacturers are blending it together, I don't see any reason why you can't do the same thing in your crankcase if you want to.

RAY: So you certainly can go back to the synthetic blend next time. Or, you may want to stay with the full synthetic. It's great stuff. It is more expensive. But because it lubricates so well and doesn't break down as quickly as conventional oil, you don't have to change your oil as often.

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