Low Oil Pressure - Synthetic or Mineral ?

I run a Classic FF1600 Kent engine.
I have run Valvoline Syn Pro since I started with the Ford and never had any trouble (45psi hot and never drops off). I spoke with Valvoline when I first started and they said if you can maintain the pressure with the thinner synthetic then use it. Well this year I bought a second engine which has a front mounted TWIN scavenge pump from Pace. Now I've got oil pressure trouble, I can't get much more than 35psi hot and at the end of a race on tickover it drops to 5psi. Also it is very rev dependent, which I've never seen before. I was pretty worried about this but talking to a couple of people this is a common problem with a) Cam driven pumps, b) Pace pumps in particular.
I could change to a Race 20/50 mineral oil but would rather use the synthetic if possible.
Views anyone ?

Hi Tony,

This isn't intended to sound condescending, but has the new engine been built (a) by a 'race' engine builder, and (b) has the oil pressure relief valve stuck off it's seat?

I deal with a couple of builders who insist that the crank grind should be slightly 'loose' to reduce friction. In my experience, these builds don't last too long!

The oil pressure relief valve problem is best indicated by the 'rising with revs' situation. This indicates that the pump and oilways are capable of sustaining a pressure, but the rapid deterioration suggests a rapid pressure loss - the valve may be the culprit. It may be that the valve needs reseating, or the spring has weakened. Try shimming the spring - CV joint balls are favourite for this!!

Hope this helps.

Sparky.

Check also that the pump is driven at the proper speed !

I've also seem partially collapsed oil filters choke off delivery enough to give these sort of pressure readings. Also don't use any of the new 'high efficiency' filters that have an internal pressure relief.

Stick with the Valvoine - it's the best there is.

Hi Guys - slight delay to replying due to needing a new password, but back now.
Thanks for the input. I know the pump is correct for the job and therefore driven correctly, yeah the Valvoline Oil seems pretty good.
You're right to ask who built the engine Sparky, it's a Philspeed - and yes he does build a "proper" engine. It's a good point about checking the valve seat for damage, it could be that.
I was at Brands last week and chatting to a few people it seems that it's not an uncommon problem and most people run a thicker oil. e.g. Valvoline Racing which is a 20/50 mineral oil. I would prefeer to stick with a synthetic if I can find one of the correct grade.