Toyota 2015 Prius Owner's Manual page 594

6-1. Specifications
Engine oil selection
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"Toyota Genuine Motor Oil" is used in your Toyota vehicle. Use
Toyota approved "Toyota Genuine Motor Oil" or equivalent to satisfy
the following grade and viscosity.
Oil grade: ILSAC GF-5 multigrade engine oil
Recommended viscosity: SAE 0W-20
Outside temperature
Oil viscosity (0W-20 is explained here as an example):
• The 0W in 0W-20 indicates the characteristic of the oil which
allows cold startability. Oils with a lower value before the W allow
for easier starting of the engine in cold weather.
• The 20 in 0W-20 indicates the viscosity characteristic of the oil
when the oil is at high temperature. An oil with a higher viscosity
(one with a higher value) may be better suited if the vehicle is oper-
ated at high speeds, or under extreme load conditions.
576
PRIUS_OM_OM47A17U_DVD_(U)
SAE 0W-20 is the best choice for
good fuel economy and good
starting in cold weather.
If SAE 0W-20 is not available,
SAE 5W-20 oil may be used.
However, it must be replaced
with SAE 0W-20 at the next oil
change.