Dave there's no argument here, you'll need to look in another thread. 😉 Thanks for outlining your understanding. It doesn't align with that of FCP - a very successful company that has been doing this as their day jobs for decades now - and we're fortunate they took the trouble to write it down.
Quoting https://blog.fcpeuro.com/the-great-debate-genuine-parts-vs-oem-vs-aftermarket I sure trust that FCP are okay with linking and quoting their blog post here. I think they have this sh*t totally covered. Some here may find it instructional.
Is there really a difference between Genuine, OE, OEM, and Aftermarket parts? The short answer is YES! Since not all parts are created equally, how should you decide on what to buy? Let's break it down.
Simply stated, these are parts that were originally installed during production of the vehicle. There are several reasons someone may want to install Genuine Parts - they want to keep the car original or that they know parts are guaranteed to be compatible. There is no need to visit your local dealer to get these parts. FCP Euro offers these parts on their website, at more competitive prices when compared to your local dealer.
When a manufacturer makes parts for the production vehicle, they oftentimes create the same exact part, place their own label on the product, and sell it under their own brand. These are called OE or Original Equipment parts. Other than labeling, these parts are typically identical. Some of these OE parts are brand names such as Lemforder, Bosch, TRW, Sachs, Lemforder, Siemens VDO, and Valeo among others. Since these parts are the same quality as most genuine parts, they are a popular option amongst DIYers since the cost savings can be substantial in many cases.
Here is an example of Genuine vs OE sway bar links for a Volvo C30. The Genuine Volvo part is 2-3 times the cost of OE, yet it is likely that it was made on the same exact line as the OE.
OEM parts are produced by a manufacturer that meets the high quality standards and produces at least one part for a vehicle manufacturer. FCP Euro recommends OEM replacement as one of the best options for quality and price. Typically all OEM manufacturers adhere to stringent quality control and are known to be exceptional quality. FCP Euro has completed decades of research and we are very stringent with the term “OEM”. Rest assured that every product we list as OEM will be of equal quality to that of OE or Genuine parts we sell.
As I stated earlier, not all parts are created equal. Aftermarket parts are created as an alternative to OEM or Genuine Parts. In some cases, such as clips, screws, fasteners, etc, the parts are virtually indistinguishable. In other cases, aftermarket parts are considered upgrades from their OEM & Genuine equivalents. In other cases, some aftermarket part quality can be lacking. FCP Euro offers a lifetime replacement promise and a guarantee that we wouldn't sell anything that we wouldn't put on our own cars. You can trust that the aftermarket parts you buy from FCP Euro are of equivalent quality and often quite affordable. If concerned about the fit of an aftermarket part, my rule of thumb is if the component being fixed is prone to leak, or it's a major job to replace, go with OEM or OE. Rear main seals and head gasket jobs aren't even that much fun the first time.
As I mentioned, even though these aftermarket companies don't supply parts to the vehicle manufacturers, it doesn't make them low quality companies. FCP Euro carries brands such as Meyle and Febi that meet or exceed OEM specifications.