I have to agree that manufacturers of equipment do NOT make most of the individual parts.
Toro does NOT make the Kohler engine, for example, nor do they make the Tecumseh transmission OR the belts OR the blades OR the bearings OR any of the little knick knack clips and fuses and grommets and spacers and bushings and switches and clicks and crap.
They don't even make the oil filter or the air filter, sure as heck not the spark plug.
So, is the Champion RJ-12YC the best plug for the Kohler?
NO! The autolite double platinum plug is way better (but YOU have to cross-x it, and buy at the autoparts store).
But the Autolite is no more aftermarket than Champion, they're both OEM for that matter.
What I would consider aftermarket is cheap / replacement parts that don't stand up to the test.
This much I learned when, after getting tired not only of paying $1 / grommet for rubber grommets for my Toro but never being able to get more than 2-3 at a time (these things break all day long, you really need at least 50-100 for some peace) and the folks never would sell me more than a few...
So, I did the research and found the right rubber grommet and ordered 100 (minimum order lol) from a big rubber products mfg (now that's OEM) for around 40 cents each or so.
They're at least as good as the original, they also last anywhere from 10 minutes to a week or three.
So, why not?
You save money, get at least as good of a product, and usually you go through less hassle once you're past the educational experience.
Then, if you have stock built up (100 grommets lasts 2-3 years), no more trips to the store for that one stupid thing you need just right this minute equals more time saved (+fuel + wear + tear).
Research is the key here.
It doesn't pay the first or second time, you spend HOURS doing research to save a few lousy bucks...
And you will make some mistakes, so start by buying a test-size sample (if you can, order only 1 or 2 to test), then if it works out, order BIG.
So over the years, you keep doing your research and remember what you've learned, that's how it pays for itself in the long run.
It's like paying yourself to become educated, experience helps.