How quickly should oil pressure come up?
However I don't think the difference oil breaking down would result in such a large viscosity changeat 40 or even 70 * F that the oil volume delivered to be noticibly less.
When oil breaks down the viscosity is still usually within the SAE standard for that grade. Changing grades, for example the difference beween a 10W-40 to a 10W-30, would result in a bigger change in viscosity.
For example, using Havoline Conventional Oil.
At 68 F the 10W-30 had a viscosity around 195 cSt compared with 10W-40 at nearly 287 centiStoke.
Keep in mind that even 195 cSt is still much thicker than viscosity at normal operating temperatures.
Between 180 and 220 F the viscosity is around 8 to 20 centiStoke.
7 cSt is probably getting too thin, so if the engine oil was runnng that hotter than 220, I'd go to the 10W-40.
Another clue to too thin will be a decline in oil pressure at all rpms.
Conversely if the engine tends to be frequently run with oil temps around 180 F, the 10W -30 will flow easier and probably provide higher volume throughout the engine. A clue for too thick is higher pressures at low rpms, but then somewhat flatlining instead of increasing with rpm. That would be the pressure relief opening, or opening further because more volume simply can't be pushed through.
The fact that a little cranking brings the pressure up faster suggests the problem may be the anti-drain back valve in the oil filter isn't sealing, or some other small leakage allowing some of the volume of oil normally trapped in the passages to drain out.
Again my reaction is the same as RRR, 10W-30 may provide better flow resulting in slightly lower idle pressure, but increasing pressure that will continue to match increasing rpm and allow increased volumetric flow at higher rpms. Worth experimenting with. Depending on the valve spring rates, and how hot the engine oil gets, not sure anything is gained with the Hot Rod oil. That's whole other discussion.
If the change in noise and start pressure occured with the change in oil grades, that's probably the answer. If not, then look at oil retained in the system at shutdown.
You could send it to one of the companies that does oil analysis (such as Blackstone Labs).
However I don't think the difference oil breaking down would result in such a large viscosity changeat 40 or even 70 * F that the oil volume delivered to be noticibly less.
When oil breaks down the viscosity is still usually within the SAE standard for that grade. Changing grades, for example the difference beween a 10W-40 to a 10W-30, would result in a bigger change in viscosity.
For example, using Havoline Conventional Oil.
At 68 F the 10W-30 had a viscosity around 195 cSt compared with 10W-40 at nearly 287 centiStoke.
Keep in mind that even 195 cSt is still much thicker than viscosity at normal operating temperatures.
Between 180 and 220 F the viscosity is around 8 to 20 centiStoke.
7 cSt is probably getting too thin, so if the engine oil was runnng that hotter than 220, I'd go to the 10W-40.
Another clue to too thin will be a decline in oil pressure at all rpms.
Conversely if the engine tends to be frequently run with oil temps around 180 F, the 10W -30 will flow easier and probably provide higher volume throughout the engine. A clue for too thick is higher pressures at low rpms, but then somewhat flatlining instead of increasing with rpm. That would be the pressure relief opening, or opening further because more volume simply can't be pushed through.
The fact that a little cranking brings the pressure up faster suggests the problem may be the anti-drain back valve in the oil filter isn't sealing, or some other small leakage allowing some of the volume of oil normally trapped in the passages to drain out.
Again my reaction is the same as RRR, 10W-30 may provide better flow resulting in slightly lower idle pressure, but increasing pressure that will continue to match increasing rpm and allow increased volumetric flow at higher rpms. Worth experimenting with. Depending on the valve spring rates, and how hot the engine oil gets, not sure anything is gained with the Hot Rod oil. That's whole other discussion.
If the change in noise and start pressure occured with the change in oil grades, that's probably the answer. If not, then look at oil retained in the system at shutdown.