Viscosity is resistance to flow. Temperature does affect viscosity, but how it affects it depends on the oil.

In straight-grade oils that have a single numerical viscosity rating (eg 100W), generally speaking the oil gets thinner (lowers in viscosity) as the temperature rises. This is not true for multi-grade oils, in which the viscosity is lower at lower temperatures, and gradually increases to the maximum value of the oil. These are displayed as two numbers (eg 20W-50), with the lower, first number representing the cold viscosity and the second higher number representing the operating value at temperature (still a variable).

Where a viscosity value is given, such as 100W, it's the same viscosity as 50W oil. 60W and 120W are oils for radial engines; they're the same viscosity. With lower values, such as a 50 weight oil, 25 is not the same value. Always verify that the specific oil that's being put in the engine is acceptable for that engine, and acceptable with regard to what's in the engine. Some manufacturers require that if a different type, grade, or manufacturer of oil are not acceptable; if one wishes to put in a quart or pint of something different, the engine must be drained and flushed. Always verify.

Lower viscosity oil tends to have lower oil pressure, but that's not always the case. For example, multi grade oil has lower viscosity when cold, but may have higher oil pressure until oil flow has been established. Oil pressure is partially a function of temperature, but a function of the resistance to flow within the oil system in the engine. Sometimes a correlation can be made between temperature and pressure to verify that one is really seeing a low pressure condition or high temperature state (high temp, look for lower pressure, etc), but don't count on it. Systems compensate, and there are things that can occur in the engine (such as debris trapped at the relief valve) that can cause pressure drops or incorrect temperatures, that aren't associated with low oil, low pressure, or high temperature).

What is important is an abnormal condition; if you see high oil temperature or low pressure, regardless of the actual cause, it's a good time to get on the ground and sort it out there. Both high temperature and low pressure (and high pressure, incidentally) can be warning signs of very serious conditions that could lead to engine failure or worse; oil is the life blood of the engine and tells a lot about the engine.