I recall a few things from building and testing Indycar motors, back when they were running the Olds and Nissan motors, late 90's. We worked with Royal Purple who were sponsors of the team.
They formulated a "special blend" for us to run the motors in on the dyno before the first pull. Literally about 5 - 10 mins of warmup until oil temps stabilized, then a couple hard pulls (to seat the rings), then oil change to synthetic, and run it in anger.
- Don't use synthetic during the run in, not because it is too "slippery", but because it is too expensive to run the synth for only a few minutes and then change it. Turns out the "special blend" was dyno oil with the purple dye. Basically you are using the warm up oil to flush and clean the engine of the assembly lube and any particles that were missed in cleaning
- Having run marine and heavy diesels, I typically prefer Shell Rotella, I also use the Royal Purple blend in the F350 but only because I get it at such a discounted price
- As 87manche posted, use the cylinder pressure to seat the rings. Cannot recall the exact figures but on a Haybusa motor built for a P2 sports racer, there was a marginal but noticeable power difference between a motor that was run hard from the very first pull on the dyno, compared to a motor that was never run hard at all, like about 5hp on 180hp motor
- My buddy is one of the best Formula Ford motor builders in the US, he warms up the motor on the dyno, usually 10 - 15 mins, then straight into hard pulls, changes the oil, crates it and ships it
- Pull the oil filter, cut it open and inspect for any shiny particles, or use a magnet over the pleats to see if you have floating particles of death
- I have been a big believer in oil samples prefer Titan based in Denver CO. For the diesel transporter on the race team, we used to sample the oil and use that as a guide on oil changes. Using Rotella, we were able to double the service interval. used to be $10 per test if you pre-purchased 10 tests, think it has gone to $15 per
Nice work Ollie
) and the rebuild is as a result of an incorrectly fitted oil filter o-ring causing it to dump it's oil and it got worse from there. The previous owner was your typical young BMW and his first thought was not stop when it started running like a bag of nails but try drive it to the shop 
