I bought a car what is dual fuel. It would not start on LPG unless the
> motor was warm. Otherwise, you had to start on petrol, and rev over 1500
> RPM before switchover. I changed the spark plugs, leads and air filter -
> that helped none.
> Next I put in a larger battery (from a V8, mine is a V6) and changed the
> oil to 5W40. The owner handbook says use 20W50 or 10W40, if you live in
> cold climate. Now she will start cold on LPG. I hypothesise that now it
> cranks over slightly faster, permitting starting on LPG.
LPG has always been hard to start on when the engine is cold. Here in New England, everyone I knew of who had dual fuel cars would start in gas and run the car (or, more accurately, truck) on gas until the engine warmed up.