The first thing I would do is ditch the breathers and install a proper crankcase ventilation system with a catch can. It can be a PCV system with breathers on one side and the other going to the carb plates above the blower or it can be an exhaust evacuation system (using check valves to regulate flow of ambient air and prevent exhaust reversion) if the exhaust is free-flowing enough. I've used both. The crankcase needs to breathe, not just vent.
I would continue using Amsoil Z-Rod 20W-50 or a comparable high ZDDP, low volatility oil like HPL HDMO 20W-50 or Driven DT50 15W-50. The low (relative) ZDDP in VRP could cause bigger problems for you than oil consumption given the flat tappet cam. Another option would be to supplement one quart of Z-Rod with HPL Engine Cleaner SAE 40. It's a fully formulated ester-based oil designed to slowly dissolve carbonaceous deposits. It's fully compatible with the oil you're adding it to and won't dilute the viscosity substantially. It also contains similar ZDDP concentration as Z-Rod so your anti-wear protection won't be diluted either.
Do not add Seafoam to that oil and crankcase. It's nothing more than very light pale oil with some isopropyl alcohol and light naphtha. The bottle it comes in is more valuable than the product itself. The boiling point is far too low to survive the heat around the pistons and rings, much less clean anything there, and even if it could survive that environment, it's not strong enough of a solvent to remove those kinds of deposits. Furthermore, the isopropanol and light naphtha (the 2 "cleaning" agents in Seafoam) react negatively with ZDDP, neutralizing it, which isn't going to do your flat tappet cam any favors. It would be like giving your engine cancer to stop a nose bleed.
You need a more polar solvent with a high boiling point (>500°F) to take the heat that won't sacrifice lubricity, won't compete with anti-wear additives, and is oxidatively stable to not contribute to coking itself. (ie: adipate ester)
After saying all of this and thinking it over... I don't think changing the oil is going to fix this issue. Given the rate of consumption, and the fact that you've been using an oil that tends to run clean, I don't think you have coked/stuck rings. Are the spark plugs oily? Seeing any blue smoke from the exhaust? Can you see the crosshatch on the cylinder walls with a bore scope? Do compression and leakdown numbers look okay? I would be investigating deeper.
