If I remember correctly, the Huey's used synth oil & the crew chief pulled oil samples and sent them back to the States. The results were used as the basis for oil changes. The thing to remember, turbines are a bit different than diesels with regard to lubricants & their contaminants.

I like synth oils, but I get good results for winter starts w/dino oil, anti-gel, block heat & a really BIG battery.