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I get it at about $17 US a gallon-about the same as Mobil 1.
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That is a terrific price and I hope I can get it for that price.

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And is Delvac 1 too thin @ 5W40?
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I do not think so. Now MB requires Mobile 1 0W-40 in all new cars and recommend it in older cars. To me, Mobile Delvac 1 5W-40 is still better than Mobile 1 0W-40. It is still a 40 weight oil when hot.

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Royal Purple Long-Rider synthetic from a truck stop is about $9/gallon
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Again, I wish I can get it for that price.

Now let me tell you a personally story about Royal Purple oil.

First of all, it is an excellent oil. In our college we have a heavy equipment department and my neighbor teaches there. He has two diesel trucks (Chevy) and has access to his lab for oil analysis. At one time he ran a batch of Royal Purple 15W-40 oil for 26,000 miles. Yes, 26,000 miles with analysis every 5,000 miles. The oil was still good at 25,000 miles (viscosity, soot, contaminants, etc) but he chickened out and changed it.

We are all too conservative RE oil changes. I think he could have gone 26,000 miles with either Chevron Delo 400 15-W40 or Delvac 15W-40. Both oils are called the million mile oils.

The only problem with Royal Purple is the use of the word synthetic. I have talked to a re-seller here and he will not say that the Royal Purple uses a synthetic base stock. Many oil experts believe it is a synthetic blend oil (except a race oil they sell) with an excellent additive package.