The two numbers associated with the oils refers to their viscosity. (the
thickness of the oil) The lower the number the less thick it is. The
first number (5W) refers to it's properties in cold weather 5 is less
thick than 10, so in warm weather you should be ok. The second
number refers to the viscosity of the oil when hot, engine running. 20
is thinner than 40 and flows much easier. You engine is much older
and no doubt has some wear on it. You may see a little smoke from
the tailpipe using 5W20 as thinner oil may slip past the rings and
cylinder wall and be turned, and a slightly noticeable reduction in
power. (loss of compression) with thinner oils. 5W20 oils were only
developed within the last 5 years or so and are called energy
conserving oils. Engine components on newer engines are machined
to much closer tolerances, requiring thinner oi,l to increase MPG (cafe
figures)
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