Just a little in site into LPG fuel.
1. LPG fuel is either Butane or Propane gas or a mixture of both.
2. Typically, Propane is used in places north of the Mason-Dixon line as it is a bit less like to freeze up when it gets "**** cold"- like maybe 10% less. Butane is commonly used below the Mason Dixon line where it is warmer and when it heats up above say 72 degree (STP or Standard Temperature & Pressure) there is less oxygen in the ambient air for complete combustion.
3. LP fuels do not use an Octane rating like gasoline, they use a thing called BTU (British Thermal Units). This can be a bit tricky to understand but don't blame me blame our "friend and former landlords" the British.
A BTU a measurement of heat created by burning (e.g combustion). Propane gas has a different rating than Butane and they are as listed below along with BTU values for gasoline Diesel & Natural gas. Note these are close approximations will and when the ambient temperature and barometric pressures rise above or above 70F & ~30 barometric pressure. This is old school (data from my Fuels & Lubricant text book in use in 1966) but it was new to me when I was younger.
Propane: 92,500 BTU/gallon or 2570 BTU/cu.ft
Butane: 102,600 BTU/gallon or 3260 BTU/cu.ft.
Gasoline: 135,00 BTU/gallon or N/A
Diesel: 139,200 BTU/gallon or N/A
Natural gas: N/A or 1075 BTU/cu.ft.
A nice conversion factor there are 4.24 lbs. of propane in one gallon.

Simply stated the higher the BTU value the more power per per cylinder /per power stroke of the engine.

Now I said all that it might be possible the tar build up issue could be related to the type of LP fuel a supplier is providing. In our capitalistic society it is not uncommon for suppliers to "short sheet" customers. In that they are providing a low cost fuel which I would think would be propane - less power might equal lower cost. But butane based LPG might be better suited for the. area and will vaporize faster & = less tar build up. Check it out at your or your customer's end.

The info on natural gas might give you some insight into why the CNG phenomenon for lift trucks didn't take off like a rocket ship. When compared to LP fuel NG has significant less BTU than either Propane or Butane. CNG powered truck are noticeably sluggish (aka less productive).