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You have already made the decision and drained the engine but to add a couple of points.
First, the viscosity was not an issue. An SAE50 engine oil and an SAE90 gear oil have very similar viscosities, of the order of 18 cSt at 100C. There are two separate SAE viscosity classifications (J300 for engine oils, J306 for gear oils).
The important issue was the gear performance level of the gear oil. If it was GL-4 or GL-3 - suitable for most manual gear boxes - it would not be any real risk. If it was GL-5 - full EP, suitable for final drives with offset hypoid gears - it could be corrosive to bearings. Safest to change out that sort of contamination. FM is not an issue.
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