Porsche 928 S 1986 Driver Manual page 95

Engine Oils
Use only engine oils which have been tested
and approved by PORSCHE. Your Porsche
dealer will be glad to advise you on the correct
type of oil for your engine.
All current engine oils are compatible with each
other, i.e. when making an oil change it is not
necessary to flush the engine if you wish to use
a different oil grade. Since, however, each
brand of oil has a special composition, you
should, if possible, use the same oil if it be-
comes necessary to top up between oil chan-
ges. PORSCHE engines have long intervals
between oil changes. You can make best use
of these long oil change intervals by using
multigrade oils since these are largely inde-
pendent of seasonal fluctuations in temperature.
92
If your vehicle is used frequently in stop-and-go
traffic in winter, the engine will not always be
properly warmed up. Condensates from pro-
ducts of combustion may accumulate in the oil.
In this case, it is advisable to change the oil in
spring so that your engine once again has a
100 % efficient engine oil.
Engine oil performance class
Engine oil is not only a lubricant, but also ser-
ves to keep the engine clean, to neutralize the
dirt which penetrates into the engine through
combustion and to protect the engine against
corrosion. To perform these functions, the oil is
provided with additives which have been spe-
cially developed for these functions.
So-called mineral oils are produced directly
from crude oil. The oils can be further refined
(hydrocrack oils) or totally converted (synthetic
oils) through a number of chemical processes.
These oils are structurally more efficient and
require fewer additives than simple mineral oils.
The efficiency of an oil is expressed, for ex-
ample, by the API classifications which are
divided into categories " S " and "C". The de-
grees of quality are expressed by final letters
in alphabetical order. The requirements for
PORSCHE engines are API class SE/CC to
SF/CD.
Viscosity
Like all liquids, engine oil is viscous when cold,
and thin-bodied when warm. The viscosity of
an oil is expressed by its SAE class. For cold
viscosity (measured at temperatures below
0° C) the SAE class is given as a number and
the letter " W " (as in winter); for hot viscosity
(measured at 100° C) the SAE class is given
only as a number.
The viscosity of an oil is, therefore, always the
same if it has the same number of an SAE class.
E.g.: A 10 W-30 oil and a 10 W-40 oil have the
same viscosity when cold (below 0° C);
when hot (at 100°C) the oil with the num-
ber 30 is thinner than the oil with the
number 40.
Single-grade/multigrade oils
Oils with two viscosities are called multi-grade
oils; oils with only one viscosity are termed
single-grade oils.
Single-grade oils can only be used for the
narrow temperature range identified by their
SAE number; multigrade oils cover a wider
temperature range (see chart).
Low-viscosity oils
Low-viscosity oils reduce internal friction in
the engine. PORSCHE approves only low-
viscosity oils which are structurally so stable
that they can be used in PORSCHE engines
both as summer and winter oils (see chart of
areas of application for oils of different visco-
sity). These conditions are currently fulfilled
by synthetic or hydrocrack low-viscosity oils.