Chapter 5 ENGINE OIL
5-4
Service limits of engine oil
Engine oil degrades through the use and by lapse of time.
Be sure to renew the engine oil if any of the properties below exceeds the limit.
Definition of properties of engine oil
Viscosity
Viscosity is a basic physical property of engine oil and
is considered as the most important aspect when eval-
uating oil.
Contamination of oil by blow-by gas and deterioration
of oil by its natural aging degrade the performance of
viscosity, which will cause the deposition of sludge
inside the engine and oil filter clogging. Contamination
of oil by fuel and sheared molecules of viscosity index
improver in oil also degrade the performance of vis-
cosity, which will cause insufficient lubrication and fric-
tion/wear of engine parts.
Total base number
Total base number (TBN) shows the ability to neutral-
ize acids such as organic acid due to engine oil oxida-
tion, or sulfurous or sulfuric acid due to the sulfur
content of fuel.
Because TBN indicates the amount of dispersant
detergent in oil, it can be used to estimate consump-
tion of basic dispersant detergent. The ability to dis-
perse sludge declines as dispersant detergent is used
up.
Total acid number
The total acid number in oil increases as the organic
acid is being derived by the engine oil oxidation, or
sulfurous acid or sulfuric acid derived by the combus-
tion of sulfur content of fuel, or the oil becomes con-
taminated with imperfect combustion products. An
increase in the total acid number will result in corro-
sion or wear of the inner parts of the engine (such as
cylinder liners or metal) due to sulfur content, and pis-
ton ring seizure due to sludge.
Water
Water in oil promotes corrosion/wear, and decreases
lubricity in sliding parts.
Flash point
The flash point is lowered by contamination with fuels.
Flash point is measured to check the dilution of fuel.
The dilution of fuel reduces oil film, and causes insuffi-
cient lubrication that will cause friction or wear of
engine parts.
Insolubles
Insolubles include acid products of engine oil, imper-
fect combustion products, sludge or soot, metal abra-
sive particles and dust. Insolubles are an indication of
degradation/contamination of oil.
Dispersant detergent, which is an additive in engine
oil, absorbs sludge particles, and disperses them as
fine particles in oil.
Total insolubles density and remaining dispersibility
can be obtained by measuring insolubles and coagu-
lated insolubles (using chemical specialities to stop
action of disperse detergent and to collect the sludge
dispersed in oil) by which piston ring seizure or pre-
mature wear can be prevented before it occurs.
Table 5-1 Service limit for engine oil properties
Property Service limit Test Method
Viscosity cst@100°C [212 °F] Within +30 and -15% of new oil JIS K2283
Total base number mgKOH/g
2.0 or more with hydrochloric acid (HCL) method
1/2 of new oil or more with perchloric acid (PCA) method
JIS K2501
Total acid number mgKOH/g Up to +3.0 of new oil JIS K2501
Water content Vol % 0.2 or less JIS K2275
Flash point (open cup) °C [°F] 180 [356] or more JIK K2265
Pentane insolubles Wt % 0.5 or less
Comply with
ASTM D893
Pentane insolubles
coagulated
Wt % 3.0 or less
Comply with
ASTM D893
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