Also worthy of note is that a well known manufacturer of all sorts of auto drive belts in Scotland specifically excludes any oil or luricant contamination in its warranty t&c's.

Most oils and lubricants degrade the material integrity of cam and other drive belts. Belts known to be contaminated should be replaced at the earliest convenience.

These belts are supposed to run dry. When replacing a belt it is permissible to clean up pulleys with isopropyl alcohol and allow to dry off.

Probably broke the following week.

Not probably. Possibly, but unlikely.

OP. belts are cheap and have a finite life. Rubber hardens and cracks.....

They are indeed, it's the labour and hassle that's costly. Are there still many people out there who seriously think that rubber is a major component of cambelts?

...here is a link to "Grako Sprazon"

https://trademarks.justia.com/716/31/grako-sprazon-71631286.html

Its a leather belt dressing and clearly not intended for use on cam belts.

Clearly this is from a legal source. Lawyers are not noted for their scientific knowledge or even accuracy on occasion in my experience. Its (sic) not just a leather belt dressing, on the can it CLEARLY states 'leather, rubber, synthetic and fabric'.