Yeah, 50W aviation oil should be a safe bet and not to troublesome to find in Europe. Don't think the 58, 75, or 105 ever really stood for the real weight of the oil. An old dealer once told me it was a marketing ploy to make it more difficult to buy oil from other suppliers. Don't know if that's true or not, but those numbers don't seem to be used until the late 60's or so.
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