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This run-in procedure is a combination of what different folks have
posted on the Mini Mailing list. Some of these points can be omitted,
but the practice is not advised.
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1. Crank the engine with the coil HT lead disconnected until you have
oil pressure. Because this can take a while, try to have a second
battery handy, or perhaps a battery booster. Some people also advise
on putting some vaseline into the oil pump so that it will suck better
in the beginning. A word on oil pumps. You should check your new oil
pump for swarf left over after machining. Often times even a new pump
can have a bit of dirt left inside. It is good to use high-powered oil
pumps, and always try to replace with a new one if the engine needed
a rebuild.
Disconnecting the fuel pump or clamping the fuel line is also a good practice as the engine will not draw fuel for nothing.
If you like a bit more hassle, take out the spark plugs. There will be much less resistance for the engine to turn, hence it will turn faster and the oil pressure should come on a lot faster too. This will make it unnecessary to disconnect the fuel pump as the manifold will not be pressurized, and the engine will not draw fuel.
If you are VERY thorough, you can prime the accessible oilways with oil so that less space has to be filled by the pump. This helps increase the pressure faster.
2. Reconnect the HT lead and start the engine. Do not let the engine speed fall below 2500 RPM for 20 minutes. This beds in the cam and cam followers.
3. Shut off the engine change the oil and filter. This gets rid of the assembly lube and any swarf left from the build.
4. For the first 1500 miles vary the engine RPM. Do not drive with a constant engine RPM for more than a few minutes (some might say for as little as 30 seconds) and do not use the engine at high RPM (say above 4000). This is the hard part and also the most important part since it is responsible for bedding in the piston rings.
The suggestion by Wade Cams (OZ) is to just keep the revs down (say below 4000) for the first 100 miles. The main thing is to make sure that you don't have the new engine idling for too long, just enough to sort out the idle mixture and that everything is OK.
The suggestions I have heard are to progressively increase the revs as you run it in and don't be afraid to use full throttle at medium revs, but don't rev it out till you get some miles on it...
I found that at 1500 miles my engine had lossened up dramatically... you can tell how it's going by how quickly it stops after you switch it off. This fact seems to have been confirmed by some other people, so make sure you dig your wrench again, and give all the nuts'n'bolts a good check. Remember that some of them need specific torque, so don't overthighten them!
Allow the car to rev freely up&down gears. But do not place it under more than very light load. Personally I was told 1/3 throttle only and light load for 200 miles (~3000rpm max on my car), then 1/2 throttle(~4000rpm max) up to 500 miles and an oil change.
At 500 miles I let the engine builder(50yrs old) have a go, and boy did he drive it a lot faster than I had thought you could!!! From 500 to 1000 miles he suggested I just drove it as normal but no sustained 'land speed' records just yet. Oil change then, that's it!
The numbers above will vary a bit depending on who you talk to, as will the frequency of oil change. I like to change at 500 miles some won't bother until after 1500.
I don't have much experience with synthetic oils but I don't think I would be comfortable switching until at least 3000 miles. Here's another thought. Until the rings bed the engine should use some oil. Remember to check often. Perhaps you could run mineral oil until the oil burning stops then switch to synthetic. This might also argue for not changing the oil too often so you can monitor usage.