Oil Viscosity not in the manual ...

[oj88]

and even with a 0w40

Personally... and it's just my opinion... a 20 y/o vehicle will probably be better off with a 5W-xx or 10W-xx. Reason being, the engine might actually benefit from a relatively thicker oil at cold start because part tolerances have presumably loosened up enough that an 0W oil may no longer provide enough initial protection.

Note also that motor oil takes a lot more time to warm up and reach the target viscosity. By comparison, the coolant temperature warms up significantly faster thanks to the thermostat valve, which can reach operating temp in about 5 minutes. Motor oil, however, could take anywhere around 15-20 minutes to reach normal operating temp. In other words, the oil might stay in the lower end of the viscosity range far longer than necessary.

Anyway, I'm just splitting hairs.