Any engine oil with decent lubricating properties will be fine.
I've been using 5w30, 10w30, 5w40 and 10w40 semi and fully synthetics in my 1600 and my old 1641.
Usually cheap Ve-Tech stuff from GSF when there's a discount code or the equivalent Triple QX stuff from EuroCarParts.
Found 20w50 to give way too much oil pressure when cold and still quite high when hot, hence changing to thinner weight oils, but that all depends on the wear and tolerances in your engine.
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