During the great YG30R dearth, at the beginning of this year I learned some things. I was needing to add antifreeze (turned out to be those bad hoses) and could not find Yanmar coolant. The manual allows 19 named alternative coolants, but no mixing. After contacting all distributors in the US, I learned that the coolant would be available within 5 weeks as it was being shipped in. I also learned you can't send the stuff by UPS air as its considered "hazardous," so getting it has to be by ground. I had borrowed all the YG30R I could from Frank of Ainokea, who graciously loaned my his spare 3/4 ga.
Anyway. I sought help from Richard in RT parts as unless I flushed and replaced coolant I could not use the boat for five weeks+time to UPS the stuff to me. I refused to flush out coolant in a boat with under 20 hours on it. BTW YG30R is near $20+ per gallon. For free, by ground, the factory sent me a gallon of coolant, but it was Chevron! I challenged this, but Andrew checked with Cascade (WA's distributor) who reportedly checked with Yanmar and I was told it was OK to mix this stuff in with Yanmar coolant, which I did.
I wonder if Yanmar would have given me a written statement that this would not void my warrantee? The IRS will guide you in doing your taxes, but not guarantee their own advice–so I wonder... It "seems" that the best bet is to use Chevron if you need something to mix with Yanmar, as it can take a week to get by UPS ground from the distributor or more if your local mechanics run out. I believe Chevron only comes in 50/50 mix, just like YG30. I took advantage of the hose replacement recall to flush and refill my engine with YG30R which is paid for under the recall. I also carry 6 spare gallons of YG30R under the sink, now, which helps balance out that port-side list.