Thanks to a tip from Hillberg over in this thread, I've discovered that the US company Petroleum Helicopters, Inc. of Lafayette, Louisiana, actually obtained and operated an Mi-8 and an Mi-10 for a short period.

In December of 1967 it was revealed that an air transportation company in the Netherlands purchased the helicopters in 1966 for resale to PHI in March of the following year, to which they were used for supporting oil rigs in South America after thorough testing by American specialists. Their usage was short-lived due to spare parts and general maintenance support being poor, as this was during the Cold War era when Russian equipment was difficult to obtain, resulting in the Mi-10 eventually being scrapped in South America. No word yet on what happened to the Mi-8.

I was only able to find the below scans about this from an eBay listing:

https://www.ebay.com/itm/1967-Press-Photo-Soviet-Union-made-helicopter-said-to-be-worlds-largest-/372916722315?_ul=BY

Does anyone have more info and possibly more photos? I'm really fascinated with this.