GL1 = 75w140 ~ winter weight -75 being the thinnest.
GL5 = 80w90 ~ summer weight - being they do not change much when hot.
So, all the difference is, being a weight factor, i.e., viscosity. Could you get away with both? Yes.
Will the syn slip the clutch? Yes. For awhile.
Would the non-recommended oil harm the trans? An emphatic no.
Would any oil keep an engine running? Yes.
So, what you are saying is that if my oil is thick in the winter and takes a little longer to reach the moving parts, we have a few hours of a wear factor we add up how many times the engine started cold?
Wouldn't there be oil on the parts when the engine is turned off? Yes.
So in other words, no harm no foul we use any weight oil, show me the winter engines with 20w50 in them and I'll show you the same performance with 5w30 someone forgets to change out the winter/summer oil and oops, I have to buy a new car each year. LOL
JMO