On the whole "5W30 not thinner than 10W30" at normal temperatures:
Varying by brand and exact product line, 5W30 in general is more readily burnt in a worn engine with oil consumption issues than 10W30 is. The 30 is the same, so why would this be?
This is a phenomenon I've noticed with my own vehicles, and thought had to be impossible, and then is something I've heard other people say as well, which caused me to do some searching on the topic. As I understand it (which is limited), there is a volatility factor called NOACK which relates to how easily an oil can vapourize. This volatility gets worse the wider the spread is between the two viscosities (e.g. the difference between 5 and 30, vs the difference between 10 and 30).
So, for example, I have one 302 V8 that is known to drink oil in general. It's pretty happy with 10W30, but it absolutely drinks 5W30 of the same brand and product line of oil.
For reasons that are irrelevant in this discussion, it's currently running a 5W40 diesel oil, and it is absolutely annihilating the 5W40. Drinks like a fish, and much of it is through the PCV, where vapourized oil would be most prevalent.
When we're talking about an engine that is maybe burning oil (1987 is 37 years ago) and maybe has some sloppy tolerances (suspected reduced oil pressure), I'd probably be running the 10W30 and if it's noisy at cold startup, seasonally change to 5W30 at the start of winter, which is the "old school" approach and is probably pretty similar to what the original owner's manual says to do. Most Ford engines before the mid 90s were specced to take 10W30 unless the outdoor temperature is regularly below a certain number, at which point they were supposed to take 5W30, and I'm almost positive my 1985 Ranger manual says that.
edit: I'm aware this is the Mazda-specific subforum and Mazda may have specced differently than Ford at that time - but some of these general principles are pretty universal. It is unclear to me what engine the OP's truck has though, because I don't think there was a 2.3 in the B-series trucks before the round body trucks where the Ranger and B-series were rebadged cousins of each other.