Don't forget that all 10w/40 semi synthetic oils are not the same - the API S* primarily for petrol engines ratings varies between oils of the same viscosity. Engine oil for diesel engines has C** rating. Gear oil has GL ratings. I would use oil in the Omega of the same API S* category / rating as GM's own 10w/40, which is SL from memory.
I don't know what prices you have been quoted for a replacement alternator, but when mine gave up the ghost, I took it to an autoelectrics place who fully overhauled the existing alternator for around £50. Replaced the electrical components with genuine Bosch (not cheap chinese versions!) and a new pulley bearing. Came back looking like new.
I thought Facepage (or whatever its called) is supposedly one place to find friends / family from years ago. Maybe enlist the help of a youngster to give you some pointers...!