OK, I'm actually working on my wife's Olds but there's no good GM forums and this forum has really helped with my 97 F-150. So - I'm changing the cables due to corrosion because of a leaky Delco battery. But, the replacement positive cable's second wire is a lot smaller than the original wire going to the fuse box. The replacement secondary wire is 10 gauge and the original is about 8 or 6 gauge. Will the 10 gauge handle the fuse box demand?

 

without knowing squat about the Olds that you are working on, I'd say no.

If it was an 8 awg wire, and you put in 10 awg wire, you are short changing yourself about 20% of the amp rating of the wire.

If it was a 6 awg wire, then it is short about 35% of the capacity.

How about a replacement harness from Napa or the likes. They should have the correct one for it.

If you go and get the 6 or 8 awg wire, don't get it at Home Depot. That is 90*C rated wire, which the highest temp rating for the correction factor is 80* C ( beyond this there is no correction factor as the next step is needed ).
Auto wire is a min of 105* temp rated wire.
Need to get 105* C rated wire at a auto parts place, or online.

Good luck with it.

Steve