Don't feel bad about being confused about oil..... The majority of people are either confused, or they simply don't know that there have been major changes in oil classifications over the past few years. I have always tried to make the information I have posted as simple as possible, but even that doesn't get through to a lot of people!
First off, you have to remember that when your Scout was made, there was not a big choice of products to choose from, and it was simply (for the most part 30wt detergent oil).... However things have changed and with the advances made in multi-grade oils, that is really the best way to go.
Secondly, it depends on the type of driving and the ambient conditions you are operating in. Generally, if the weather in your area is somewhat moderate, 15w40 would be my first choice, however if you experience severe winters, then I'd probably recommend 10w30, on the other end of the spectrum, for extremely hot summers, 20w50 would be the way to go.
The 15w40 wt oil is generally considered a "diesel oil," and the pre 2007 specification of ci-4 or before will have a better additive package in it than the current cj-4 oils will have, so if you wish to use the 15w40 in your engine, you should try to purchase oils graded either ch-4 or ci-4 for diesel, and they should also carry the gasoline rating of sl or previous. The newer sm oils for gasoline engines have a very low additive package and will not provide as much protection as the ones mentioned above.
You might have trouble finding ci-4 or sl oils at the place you mentioned because those specifications changed in 2007 and many oil manufacturers stopped producing them, switching over to the cj-4/sm formulas.
If push comes to shove and you cannot find what you need locally, the other choice would be to either order the correct oil from ihon, or to order the swepco 502 oil improver which will boost the additive package of the newer oils, especially zinc. Ihon also carries the 502 and it is sold by the pint bottle. One pint will treat up to 5 quarts of oil.
If you are considering either synthetic or semi-synthetic oil, be advised that most of them will contain fewer additives than the "dino" counterparts do.
Hope that clears some of the cobwebs.....
