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Only available to the Series1 RX-8s due to design changes in 2009, the SOHN Adapter is an adapter that changes the source of injection oil for your RX-8 (or other applicable rotary). From the factory, the source of oil used for injection is the 4-stroke oil that you change at regular intervals and you have to check to see consumption of periodically. This is a compromise made for consumer benefit reasons, but is certainly not the most ideal oil, as 4-stroke oil wasn't meant to be burned. The SOHN Adapter is just a plate that mounts to the OMP (oil metering pump) and allows you to supply an alternate source of oil for injection instead, also allowing you to inject your choice of oil as well. This means that instead of injecting dirty heat cycled 4-stroke that wasn't meant to be burned, you can inject clean 2-stroke that hasn't been heat cycled and is designed to burn cleanly! Costing around $150 for the pump with shipping, and plus a bit more for whatever tubing and alternate reservoir you use, it's a really cheap mod that can significantly help your engine's health.
This isn't a common modification at all, but can produce benefits with higher oil pressure from the reduced oil system complication. The main drawback is that it can be difficult to find a good point to mount the cooler, though there have been successes with a horizontal splash pan mounted one and a low one in front of the radiator.
This is growing in popularity for Series1 RX-8s, especially as our engines are dropping out of warranty. It is basically a set of shims that changes the spring strength in the oil pump's bypass, and increases the oil pressure to approximately the same oil pressure as the RX-7s and the Series2 RX-8s have. This may also be accomplished with a complete replacement moving to the FD RX-7 unit. While there are no real direct known or documented benefits, and there have been significant debates over pressure vs flow, it is oil pressures that Mazda used to have, and returned to for Series2 RX-8s.
Note: If you do decide to increase your oil pressure to the Series2 oil pressure values, make sure you use higher pressure bypass oil filters. The easiest way to determine this is to simply use Series2 oil filters once you make the switch. On a Series1, it's a tight fit, but doable as long as you don't have any sandwich plates under the filter.
Similar to helping the radiator breath, the OEM oil coolers can be helped by opening up the wheel well liner behind the oil coolers, allowing better air flow. The only drawback is in how you modify the liners. Open them up too much, and you could end up having your front wheels throwing road debris into the back of the oil cooler, which isn't helping you at all. Care should be taken in the modifications to make sure you are still shielding the cooler from the debris being thrown by the wheels.