All 1989 SHOs had a manual transaxle so any maladies affecting that transaxle would not be the same as those troubling an automatic box.
The thermostat usually fails by not fully closing. The hot water test will show whether or not it opens, but you need to verify that the moving emmber is pressed against the seal when cooled. Even a slight sliver of light that can be seen through the seating area can prevent the engine from reaching the normal operating tempetaure range on cooler days.
As long as the gauge needle is getting into the band defining the NORMAL range, the temperature is condered to be normal. The 180F thermostat originally equipped with the 3.0L engine will have the gauge needle point to the "AL" or even lower area of the range. If your gauge reads on the "L" then it is in fact normal. Mine reads just below the "L" at the 180F opening temperature of the thermostat.
The 190F thermostat supplied with the 3.2L will normally position the needle around the "MA" part of the scale.
If the needle is not reaching the NORMAL range after about 3-5 minutes of engine operation, or at times drops below the low tick mark of the NORMAL range, the thermostat should be replaced. Poor fuel mileage will result from a cold running engine if the temperature does not rise enough to allow the engine control system to enter the "closed loop" mode of operation.
Even if the temperature is correct, tired, old oxygen sensors can cause a similar mileage penalty. The O2 sensors should be replaced at the same intervals as the spark plugs or every 60,000 miles in the SHO's case. The O2 sensors will age and cause slow rich/lean switching without setting an error code or causing an illumination of the CHECK ENGINE light.
The manual transaxle (MTX) is most likely leaking from one or more of the *********** seals (in order of likelyhood): output (axle) shafts, shift shaft or input shaft. The MTX rarely leaks from the gasketless ****** mating surfaces no matter how hot it has been.
The darkening of the fluid is not a good sign. What fluid have you been installing and to what level have you been filling the casing? Since little heat is generated in the transaxle during normal operation, the darkening of the fluid indicates there is likely damage already suffered in the internal works. If a cause of the fluid darkening is not identified, a removal, disassembly and refurbishment of the transaxle in the near future would be in order before a catastrophic failure should occur.
Any other information you could provide may help us give you a better idea of what you may be facing.
