The Bentley manual does not offer info on synthetic oil weights. The oil
viscosity chart on p. 17 is a bar graph; I've typed, below, the
approximate outdoor temperature ranges in degrees specified for all the
oil grades shown in the chart (in degrees, of course):

SAE 40:                         +64 - +102 F (+20 - +40 C)
SAE 30:                         +32 - +85  F (0 - +29 C)
SAE 20/W20:                     +13 - +49 F (-10 - +9 C)
SAE10W:                         -9 - +21 F (-22 - -4 C)
SAE 20W-50 AND SAE 20W-40:      +12 - +83 F (-8 - +28 C)
SAE 15W-50 AND SAE 15W-40:      +2 - +83 F (-10 - +28 C)
SAE 10W-30                      -2 - +55 F (-21 - +14 C)
SAE5W-20 AND SAE 5W-30:         -20 - -10 F (-28 - -9 C)

No info available for other grades. I notice that their doesn't seem to
be a single oil recommendation that covers New England's three
seasons--or even just Summer; from May through September we experience
temps ranging from "frost" to 100+ F (today, 70's). Even July dips to 60
F now and then, from June to Sept. temps well above 85 F are
commonplace. Thoughts?

George
83-1/2 Westie