Pretty sure this is solved. I removed the sender and hose and cleaned out the hose, replaced the sender with a new one and changed the oil, now running SAE40 which I should have been anyway this time of year. I'm pretty sure it was the sender but in any case, several flights now and no spiking OP.
Thanks for all the responses, I'm sure you've all been on the edge of your seats wondering how this would turn out
RV-6 / O-360 / CS, 1500+ hrs, 1st flight Sept. 1999
Outstanding Workmanship OSH 2000
Airport committee chair & ASNV for Seaside, OR Municipal (56S), www.seasideairport.org
Donated August 2020