The gearbox probably needs engine oil, so I'd listen to Zastava over Haynes and use a straight forty.
Gear lube might erode some metals in the transaxle assembly.
I'd probably try Maxlife 10W-40 in the engine, which I think would probably work well.
Remember that the engine oil recs were likely based upon what was available in Yugoslavia at the time, and do not take into consideration the oils available in North America even back in the mid 'eighties when Malcom Bricklin was trying to flog these things in the US.
Gear lube might erode some metals in the transaxle assembly.
I'd probably try Maxlife 10W-40 in the engine, which I think would probably work well.
Remember that the engine oil recs were likely based upon what was available in Yugoslavia at the time, and do not take into consideration the oils available in North America even back in the mid 'eighties when Malcom Bricklin was trying to flog these things in the US.