I don't think oiling the engine is a problem
and if the viscosity of the oil is low enough to allow the engine to start
and run it is probably good to go by the time you taxi out and do the runup.
And I don't think there is a problem with mixing viscosities or brands. 

Lot's of assumptions here. Your butt is on the line. Where's the data?

 

Jeff Edwards

I can't provide any guidance on exactly when it is safe to do the runup, but it may be useful to know a little about the function of the oil system.

 

1. Oil pumps are positive displacement and should have a relief valve to prevent overpressurizing the system. While I have heard of oil filters being ruptured, I suspect that the root cause was actually a problem with the relief valve or possibly a defective oil filter. However, the pump still has to rotate no mater how thick the oil may be and I have heard of '60's/'70's Ford V-8s stripping the oil pump drive shaft after a  quick burp of the throttle at startup with heavy oil at cold temperatures. Anyone familiar with these engines will agree that this at least makes sense. (these engines required a special hardened drive shaft just to use a high pressure pump!)

 

2. For a given flow rate, the pressure drop of the oil through the filter and passages within the engine varies with the viscosity of the oil. When this pressure exceeds the relief valve setting, a portion of the oil flow is diverted back to the sump. Thus, at cold temperatures with heavy weight oil, flow rates to the bearings and valve train may be limited.

 

3. For variable flow rates, the pressure drop changes with the square of the change in rate. Since we are using positive displacement pumps, driven from the crank shaft, doubling engine rpm will double pump oil flow and quadruple the pressure drop.

 

4. The crank shaft serves as a centrifugal pump, feeding the rod bearings from the oil in the main journal bearings. Increasing engine speed increases the oil flow rate to the rod bearings and evacutes oil from the main bearings. Accordingly, the engine needs additional oil flow to ensure that the main bearing oil clearance remains full as engine rpm is increased. The rule of thumb for small block Chevy engines used to be 10 psi for each 1000 rpm to ensure suficient oil flow using hot 20w-50. Due to the configuration described above, it is possible that a cold engine may be safely operated at low rpm, but be damaged as speed is increased without first warming the oil.

 

5. To adequately lubicate the cylinder walls, the oil must be warm enough/low enough in viscosity to spread quickly upon contact with the cylinder. Cold oil that remains as a droplet will result in increased top end wear. Lower rpm's provide more time for the oil to flow. Lower cylinder pressure operation results in lower ring contact stresses until the engine warms up to allow the oil to lubricate properly.

 

The urban legends regarding mixing brands and/or viscosities of oil motor oil have been around as long as automobiles. I could never make sense of them nor find anyone who could explain them to me. Just the same, several wise engine gurus I have known and respected have preached the importance of not mixing your motor oil.

 

I have always ignored the issue. I have intentionally mixed 10w-40 and 20w-50 because I wanted something between the two, sometimes with each weight sold by a different company. I have never observed a problem with this on the track or the street. Of course this doesn't prove anything at all, it could simply mean I was lucky.

 

Additionally, at least as I remember it, synthetic oils are designed to be multiweight oils. They don't need viscosity stabilzers or detergents. These properties are built into the molecule. (can anyone verify this?) So, it seams that there would be even less issue with mixing brands or viscocities of synthetics.

 

Rob