Transfer case fluids.

My 231 calls for ATF to be used as the lubricant. However, I want the best for my jeep, so I went out and bought some Royal Purple Synchromax. On Royal Purple's web site it states that the synchromax is perfect for "Synchromax is recommended for manual transmissions that specify an automatic transmission fluid or motor oil. It is also ideal for 4x4 transfer cases and / or 2-cycle and 4-cycle motorcycle gear boxes." The question is, should I run the royal purple ? I'd rather not blow my transfer case apart.

The NP231 and NP242 use a light-duty, gear-driven pump to circulate the fluid, vice the "splash lubrication" usually used in manual gearboxes and old transfer cases. This means it requires a low-viscosity oil (like ATF, which has a base viscosity of 10 - vice gear oil, which can run from 75 to about 180. The higher viscosity mean thicker oil, which tends to stick to things longer.)

As long as your Royal Purple is rated for manual gearcases that use ATF (like most newer ones,) you should be fine. The trend away from heavier oil comes from wanting to increase fuel efficiency by reducing the work done in circulating - same reason that most newer engines want either a 5W-30 or a 0W-30 engine oil.

Based on the blurb you read and reposted, it sounds like it would be fine to me.