Im going to assume this is a medium capacity diesel generator, havoline coolant works really good for diesel engines as it has excellent anti foaming and excellent cavitation protection as well.
Using old iat green is doable but has a very short life, meaning you will have to recharge the coolant in about 2000 hours or replace it fully. With havoline you wont need any sca recharge or buying charged filters. Just pour and forget for the next 10000 hours (keep a track of its ph scale from time to time)
If you want to try something else, then look for shell or caterpillar ELC, they are formulated for such engines specifically.
Another choice is mercedes benz old fomula which was BASF G05, its really good stuff but its silicated hoat with a dose of diesel additives.
When i was in pk (more than 15 yrs ago) havoline was called caltex coolant and was also available in 20 litre canisters, it was also available at some caltex lube jobbers in 200 litre drums which the jobber would sell by the litre.
Dont know if they still offer such, you will need to call a real caltex/chevron lubes jobber/distributor to find out.
