Genuine parts made for a product are also referred to as OEM which stands for "Original Equipment Manufacturer" referring to the company that originally manufactured the product. The original manufacturer can market OEM parts under their own name or sell them to a secondary company that will use them as part of one of their products. Many times the original manufacturer will manufacture a part with the secondary company's name stamped, molded-in, engraved, or imprinted on the part. Usually when this happens, the original manufacturer has an agreement with the secondary company to not sell these parts with their name on them to anyone but the secondary company. However, the original manufacturer can make the same part without the secondary company's name on it and sell it as a "will fit". What this means for you is sometimes you can get the exact same part (by the original manufacturer of the OEM part), but under another name and at a lower cost!
Some manufacturers will copy or duplicate another company's product but due to patent laws have to make it look slightly different than the original product. These parts can also be sold as "will fit". In some cases a "will fit" part will work just as good as an OEM part. And, in some cases you will definitely want to use an OEM part.
Most toilet manufacturers buy most of their internal fill valves, flush valves, and flappers from other companies that specialize in making fill valves, flush valves or flappers. But sometimes they manufacture their own fill valves or flush valves.
For example, the older Case one-piece toilets have parts currently available, but all of their parts are genuine OEM and only available from the manufacturer. No "will fit" parts are available or made except for tank balls, tank floats and flappers.
Note that the previous example is regarding one-piece (typically more expensive) toilets as compared to two-piece toilets that are usually less expensive.
Most two-piece toilets use parts that are generally replaceable without the need to buy OEM parts. OEM parts can be more expensive than "will fit" parts and since the tank can be removed from the bowl many "will fit" parts are interchangeable with OEM parts at a fraction of the cost. There are many different fill valves available for two-piece toilets, and a fill valve is a fill valve. You just need to choose one with a length that is less than the height of the tank. Many of the better fill valves have adjustable heights to fit many different-sized tanks.
However, we have found when it comes to purchasing parts for faucets it is usually best to buy genuine OEM replacement parts for the internal working parts, unless "will fit" parts are the only parts available.
Let's consider a shower. In order to replace the stem or cartridge you will usually need to turn the water off to the entire house, unless your builder or plumber installed a shower faucet with shut-off valves located underneath the trim plate (very rare). Now this is where you will want to make sure the parts you purchase and install will be the best possible so you can get the job done right the first time.
We have experienced many upset customers who purchased "will fit" Valley faucet parts from other companies only to find they leaked after they were installed. Once they purchased the original OEM parts we offer here at PlumbingSupply.com, their problems were solved.
It's not as critical to use OEM parts for external trim plates or handles for faucets since these parts can normally be replaced without the need to turn off the water to your entire house. In many cases, if the faucet is too old you will only be able to find "will fit" trim or handles anyway.
Sometimes you may find OEM parts that were made to fit many different brand name parts. For instance, some parts that were made for Valley faucets were also made for some Eljer, Sterling and Aqualine faucets. Some "will fit" single handle shower escutcheons we sell will fit Valley but actually have the words Eljer on them.
So when it all comes down to it, which part should you buy, OEM or "will fit"? It all depends on your product, what parts are available, cost difference and which will be more advantageous for you in the long run.