Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 25Get this manual
MAINTENANCE
Gear Case Lubrication
3-14 In(83mm) Diameter Gear Case
When adding or changing gear case lubricant, visually check for the presence of water in the lubricantIf water is present, it may have settled to the bottom and will drain out prior to the lubricant, or it may be mixed with the lubricant, giving it milky colored appearanceIf water is noticed, have the gear case checked by your dealer.Water in the lubricant may result in premature bearing failure or, in freezing temperatureswill turn to ice and damage the gear case Whenever you remove the filldrain plug, examine the magnetic end for metal particlesA small amount of metal filings or fine metal particles indicates normal gear wearAn excessive amount of metal filings or larger particles (chips) may indicate abnormal gear wear and should be checked by an authorized dealer