Oil Temperature Gauges

The ideal engine oil temperature is in the range from 80°C to 120°C (~175° - 250°F). Only at 80° the water content (condensate from ambient air or engine ciold starts) will start to evaporate. Water in the engine oil will form white foamy deposits and lubrication quality will worsen. If kept above 120°C for a longer time, it will loose its chemical-mechanical qualities and change to the bad.

So, how does a Harley rider know the engine oil temperature, and where's the sump?

Air-cooled Harleys do not have an oil sump like cars do. The Harley rider's solution is to put a temperature feeler gauge into the oil tank filler hole instead of a normal dip stick. The crucial point is the permanent temperature, not the local temperature peaks at some spots in the engine. Temperature gauges are available for different models like Sportster, Softail and FLH, according to the oil tank. The gauge serves as a dip stick as well.