I have 5 of my 10 truck fleet running on propane, from a 350 GMC to a 534 Ford 880 tandem. I figure it saves me up to $1,000 on my monthly fuel bill, as propane typically sells for around 2/3 gasoline price here. In addition to reduced fuel cost, engine life is extended. One of the trucks, a 1990 351 F250 4x4 has a Vialle dual fuel system and it has been very satisfactory as the truck now has over 363,000 miles (585,000km) on it. I rebuilt the engine at 258,000 miles (416,000km) as I bent a pushrod at that mileage. It gets up to 14 mpg on propane and up to 18 mpg on gas in highway driving. I use Esso Ultra 5W30 oil, and change oil and filter every 4,600 miles (7,500) km on all of them.