The question is “When do I use aftermarket parts and when should I use OEM?” There are several guidelines or “rules of thumb” when trying to answer this question. First, let’s look at repairing your car as opposed to modifying your car.
When it comes to repairs rather than maintenance, the issue is less clearly defined. As a general rule, I would OEM (”factory”) parts for the “internals” of engines and transmissions. This way you can be sure of proper fitment and compatibility with the drive train. The one area where I will look to the aftermarket are those “replaceable” items such as starter motors, clutches, pressure plates and throwout bearings. These are also available from aftermarket suppliers with equal quality and (usually) lower cost. In fact, starters, alternators, etc. are usually “rebuilt” OEM units that are far less expensive than the new factory part.
A few more confusing facts: OEM means that it comes from a manufacturer that is subcontracted to the original manufacturer. It does not always mean that it’s the same as was in your vehicle originally. And it might not be labeled with your vehicle manufacturer’s brand. For example, a hood ornament may have been made in 2015 by the ABC Hood Ornament Company (a made up name) and installed on your 4×4 at the factory. The 4×4 builder may have changed manufacturers in 2016 to the XYZ hood ornament Company. If you go to your 4×4 dealer’s parts counter for a replacement, they can sell an XYZ ornament with their logo on it as an OEM. The same XYZ part might be available at your local parts store under a different brand but still labelled as OEM.