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Low hanging fruit in winter, in no particular order:
-LRR winter tires
-Tire pressure
-Warm air intake
-Grille block
-Block and oil pan heaters
LRR tires and high pressure are really great mods any time of the year. Beyond that, it's a matter of keeping the engine warm as much as possible. In my car, when it starts to cool off outside I pull off the plastic intake snorkel and replace it with flexible RV dryer ducting, which I routed from the intake to just under the catalytic converter. I can get air into the engine over 100F when it's subzero outside if I drive the car long enough to get everything warmed up. Warm air has a variety of benefits, but to summarize it, automotive engines are all heat engines. You're turning fuel into heat, and then extracting mechanical energy from that heat. During winter any heat you can redirect back into the engine which would otherwise be lost is like turning exhaust back into gasoline and putting it back in your tank.
Regarding the propane system, you mentioned that it "injects just before the O2 sensor" - the O2 sensor is in the exhaust, after the engine. You're definitely not injecting it into your exhaust.
The way I read it, it's injecting propane into your air intake, which is where your car injects gasoline too. Propane burns about as well as petrol in most engines, and propane has ~25% less energy content per gallon (which is to say how much heat it turns into when you explode it), so for every 1.33 gallons of propane you inject, that's one gallon of gasoline you're not burning - assuming the car is able to fully compensate for the extra fuel you're dumping into the engine. This will in fact save you money if propane is at least 25% cheaper per gallon than gasoline - again assuming the car is able to adjust its fuel maps enough so as not to run rich. Which might be why it's only really talked about in diesels.
My 2 cents: Propane is a great fuel, but the 30% savings claim is total bull. It might save you money in the long term but how much is going to be based entirely on the price ratio of propane to gasoline, and whether your engine will adequately reduce the amount of gas it burns when you add extra propane fuel. You'd effectively be substituting part of your gasoline with propane, there're no other magic savings to be had.
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