Weighty Decisions

• Engine oil weight is determined by the viscosity of the fluid.Comstock/Comstock/Getty Images
  
  Motor oils are available in different weight classifications that are determined by viscosity. Straight-weight oils such as 30W remain at a constant viscosity regardless of operating temperatures. Multiviscosity oils such as 5W20 will be at the lower, thinner weight when cold. Engine heat forces the multiviscosity oil to thicken to the upper, heavier weight. The separate and distinct properties of the two types are not compatible, and straight-weight oil should never be mixed with multiviscosity oils.
  
  

Required Reading

• Oil weight recommendations may appear on the labels of decks or motors.Comstock Images/Comstock/Getty Images
  
  Each lawn mower manufacturer specifies the oil weights most suitable for the machine in the owner's manual. Some manufacturers may also provide written guidelines on stickers or labels applied to the mower engine or deck. Weight recommendations may vary with temperature ranges anticipated in different locales or times of the season. Some simple guidelines can be followed in the absence of such information.
  
  

Old and New

• An older mower with many operating hours behind it may benefit from the use of a straight-weight oil. Constant thick viscosity may slow small leaks or minor oil consumption. Engine heat produced by friction may also be reduced in an older mower by use of a straight-weight oil. Newer machines usually require thinner weights of multiviscosity oils to provide lubrication at start-up and friction protection at operating temperatures. Oil weights above 30W or 10W30 are rarely recommended.
  
  

To the Point

• The main purpose of engine oil is to deliver lubrication to moving engine parts. Good clean oil will form a film between moving parts to reduce friction. Oil that is too thick may not reach parts soon enough at start-up to prevent wear, and no benefit is obtained. Synthetic oils are known to reduce heat and friction with lower weights. The higher cost of synthetic oil is offset by the small amount required and the superlative lubrication provided.