The objective Lens should always be kept clean. Using index-matching oil at high magnifications could leave some remains on the slide and may get on to other objective lenses. Oil, water or particles of any kind will degrade the image quality and must be removed. The lens should be cleaned only with MILD SOLVENTS (Recommended: Methyl, Ethyl, Isopropyl alcohol or distilled water).
The fan filter should be cleaned every two months; if conditions are extremely dusty clean the fan filter monthly.