Oil Temperature Gauges
One of the things you will find if you have a oil temperature gauge is that regardless of the fact that oil is running through your engine and cooling it just like the coolant - it takes on and loses heat differently than coolant.
Oil temperature is probably the most important thing to know. Free reving (or worse - racing) on an engine that has not fully reached it's oil operating temperature is extremely dangerous. But don't think that the coolant gauge will tell you this information - it won't.
The coolant temperature gauge can be misleading
As a general rule of thumb, after the motorbike is warmed up, oil tends to be a few degrees warmer than the coolant.
However, it takes much longer for oil to come up to temperature than coolant. When you start your bike in the morning, most of us are wise enough to not romp on the bike until the coolant gauge is up to operating temperature. This is certainly better than romping on it cold, but it's still not quite ideal.
You see, the oil, especially in colder ambient temperatures, takes several times longer to come up to temperature. That means the coolant temperature gauge can be misleading (the coolant might be at 90 ◦C while the oil is still 60-70 ◦C).
Oil will not get to complete operating temperature easily by simply idling, it requires driving around and putting SOME load on the engine. I see people in the pits at races all the time reving their motors to 'warm the engine up'. It won't do any good and is only putting premature wear on the bike.
The best way to get a motorbike's oil temperature up is to simply drive it around for a few minutes. Ideally you'd have an oil temperature gauge to tell you when it's at operating.
What's the danger of running an engine cold?
Total engine failure.
Well, that may seem a little extreme, as certainly all of us have run a engine that was cold, hard. We probably even got away with it due to the amazingly good engine design we have today. However, it's an extremely risky thing to do and can easily result in catastrophic engine failure. ESPECIALLY in highly tuned, built engines.
If the engine is way too cold (ie, the coolant hasn't even come up to temps), it's not making its ideal power either. VTEC engines actually do not engage VTEC unless the coolant is up to temperature, for example - this is true of many other variable valve timing technologies as well. Think of it as Honda trying to save you from yourself.
In all engines, the engine's clearances are significantly tighter, creating extreme amounts of stress on the engine's internals and the piston rings will not have properly sealed with oil temps too low.
In short - it's really bad for your engine to run it hard until it's OIL is completely warmed up. Coolant temperature is a false indicator.
What's the danger of running an engine hot?
Since motorcycle engines produce a lot of power for their weight due to the high engine speeds (a truck engine = 2,500 rpm, car engines = 6,000 rpm, modern 4 stroke motorbike engine = 15,000 rpm or more). So the circulating oil comes under huge thermo stress as it transfers heat from low mass components under high power conditions.
When the engine is working hard, oil temperature rises rapidly. The oil can vaporise leading to higher oil consumption and a fall in protection. After a while the oil can become contaminated.
So without going out and buying an oil gauge and all that, what's the big take away from this discussion?
When you first start running your bike for the day, make sure to keep the revs low and take it easy for at very least the first 5 or so minutes of driving, longer in extreme cold temperatures. Most importantly, and least obviously - don't trust your coolant gauge to be a good indicator that your engine is fully warmed up.
In motorbikes with oil coolers that utilize the coolant to cool (used on many imports, sandwhiched between the oil filter and the block), the oil temp will actually come up with the coolant as an added bonus to keeping oil temperatures cooler under high loads. In air cooled engines the oil plays a very important part in cooling.
Low oil level = higher oil temperature
For this reason it is crucial not to use your motorcycle with low oil level. If your oil level drops below the recommended, the oil temperature becomes higher (as there is less fluid to cool the same engine heat), therefore having an oil temperature gauge could be a good indicator for oil levels.
If you're putting gauges in your motorbike, you might also consider an oil temp as it can be a real tool in assessing the load on your engine and keep you aware of situations that might harm the reliability of your engine.
Remember, an engine fully warmed up but not heat soaked creates optimum power. In the real world, this means when you're driving down the highway and the engine is plenty warm and the airflow through the engine is taking warm air from the engine out through the motorbike. Never try to get a better time at the drag strip by running with cold engine oil and never try to warm a motorbike up by reving the engine.
Keeping this advice in mind will certainly keep your engine alive much longer and prevent you from having a really bad day.