The carmakers haven't used "break in oil" for decades.

I'm not sure what it consisted of, if anything different. I think the idea was probably to get you to change the first fill early, which makes sense.

I just got my Daughter's 300D back together. I had scrubbed the honing grit out of the block, scrubbed it some more, and when I got tired of that, I scrubbed it again.

After that I filled it with cheap 30W oil and a fresh filter, drove it to town and back which is about 44 miles round trip, plus a trip up the road and back. When I got back from town, while the oil was hot and everything was churned up in it, I pulled the plug and let it drain all night in an attempt to get as much particulate that may have been in the oil fro the cylinder walls and rings mating with each other, as I could.

I then put in a fresh filter and filled with my normal Chevron Delo and started driving it. I will change that at about 1,800 miles with it hot again to get out any particulate that may come from the cylinder wall break in.

I would suggest something similar to just get the honing grit, dirt that may have gotten in and lint from rags. If the block was honed and it's too late to really scrub the block before assembly, I might even drain the first fill after about 20 miles which will get it to operating temperature. This is a critical time and if there is a lot of grit, it can spoil the bearings fast.

Best of luck,