My experience is that OM tire pressures are guidelines that are typically set at the highest pressure that you would reasonably run at the loads specified. This provides the best fuel mileage, the longest tire life, the most headroom for high speed cruising without tire overheating, and the biggest load carrying capacity. But it is seldom the optimal tire pressure for aggressive twisty riding or anything off pavement.
For off-road, lower is almost always better and it is truly amazing how much better a bike like the S10 will feel in the dirt with 25-30 psi rather than 36-42. Lower is even better, but you do increase the risk of wheel damage in rocky terrain and at some point (typically around 15 psi) rim locks start to become necessary. But there's a darn good reason trials riders run 4-6 psi!
In the end, it's about tradeoffs. You certainly want to run more pressure for higher speeds and loads. And if you want to get the best tire mileage, run the max allowed on the sidewall. But airing down a bit to get more heat in the tires helps for fast pavement riding (I'd run about 30-32 for a track day in the S10), and off-road, lower is almost always better.
If you ride the S10 off pavement and are at 42+ psi, you owe it to yourself to try 30, at least for a short period. Its transformational.
- Mark