So i have lowish warm oil pressured due to running a non-VVT (thin) oil pump with a VVT head.

I'm talking 40 psi warm max.

So, I'm thinking of switching to xW50 oil, to improve the film strength.

Any downsides to this?

 

Castrol 10W60 here in all race cars. Good stuff.

 

My last oil change was 20w-50 synthetic (Valvoline), band-aid for a FUBAR'd exhaust valve. The rebuilt head arrives today, but it ran quiet & cooler with 20w50 vs Rotella T6 5w-40. Fuel mileage dropped 1-2 mpg vs the 5w-40.

I'll try to find the thread about issues with too viscous oils from BITOG, potential friction increase IIRC. Miata tolerances (PTW, bearings etc.) are typically tighter then motorcycles & rotaries, OEM 20w50 users. I'm assuming the thicker oil's minimum film thickness & slower flow might not create as constant a film?