I'm looking at doing this because one of my hoses is slightly abraided (car must have had a bad mount at some point and it looks like one of the belts scraped it). I heard that the size of the nipples is slightly smaller than 5/8" and 5/8" hose will fit sloppily.

5/8 is close to 16 mm (15.8 mm). I'd bet that 15 mm is the correct size. Here's some metric 15mm ID oil-rated hose sold for Porsches. Bit expensive, but you only need two meters or so of the stuff...

https://www.************.com/p-1724-oil-pressure-rated-nbr-15-mm-id-hose.aspx