I am still hovering around the oil pressure relief valve. It will bypass the pressure sending unit therefore give you a zero reading while the engine is really getting oil pressure.
NOW ANYTHING is possible but I think changing oil to 20/50 is good thing to do and could bring pressure up from 25 to 35, I don't think it will bring it up from zero. Also think the sending unit is not the problem because it would more than likely have a hard failure and not a slow loss of oil pressure. But don't totally discount.
One other possibility is a clogged oil filter. That would open the relief valve and the engine would receive oil but the sending unit would be bypassed.
What I suggest.
1. Drain oil into a clean pan.
2. Run a magnet around the pan and see if you pick up hunks of metal. Hunks of metal is a bad sign.
3. Pull off oil filter unit (easy to do) and inspect it and inspect the relief valve in the unit. Replace the relief valve assembly.
4. Reinstall filter assemble.
5. New oil filter
6. 20/50 oil
Run and see if the problem is corrected.