What is dieseling and is it bad?

Dieseling is the combustion of lubricating oil inside the compression chamber when the piston compresses air to roughly 2,000 degrees F per shot.

A small amount is normal in many spring guns and even contributes a little energy. Heavy dieseling produces white smoke, sharp crack-like reports, and erratic velocity, and indicates too much oil or the wrong oil. Detonation (a louder bang) can damage seals and the spring. The fix is to run the gun until excess oil burns off and to use only manufacturer-recommended chamber lubes going forward.