Power loss is less when switched to return on svo 300 SD

Aloha,
we're still fighting with power loss and figured out that if the switch (FM 100) is on return fuel, we can make it up the hill...clean filters,oil and lines. The car ran fine after filters change for ~600mls. How do I test if liftpump is bad?
I also wonder if that new Cosco Soybean oil ,green label , is recommended.(ascorbic acid preservative contained)
It is hard to find restaurants here that use non hydrogenated oils.(Who would think that, considering the prices one pays here for a " nice meal in paradise". please share your thoughts or advice thanx,Tia.