ok after using running in oil for 500k's or so I'll drop some mineral 15/50 in and then after about 2000kms or so I'll consider switching to a semi or fully syn.. 15/40

so any ideas on what to fill up the old C4 trans with? just any ATF?

Keep using mineral up to 2000K's or more. the important thing to remember is the oil change interval during the run-in period.
C4- any ATF as long as it complies with the year of your trans.

NEVER EVER put CHEAP mineral oil in a fresh engine. or any engine for that mater.

first sump full should be a a good quality mineral deisel oil. shell Rimula X , fuchs ultrlube 1540,

deisel oil handles particulates beter then oils designed for petrol engine. A handy thing concidering whats going on inside your engine during break-in.
The two listed also have the aditives missing from petrol engine oil that help to protect highly stressed components that put a alot shear force on the oil.

but doesn't the defeat the point of running it in? cheap mineral oil or running-in oil is supposed to let the engine wear

you need to bed the rings in. You still need to protect bearings, cams, lifters, rockers, cam chains, dizzy gears. and you also need an oil that can handle all of metal particals that the bed in process produces. cheap mineral can't.

with all the dizzy gear wear problems that ford V8 engine fitted with none stock cams seem to be having, it'd be best to take every step to prevent it.



the aditive that you're looking for in a break in oil is
Zinc alkyl dithiophosphate. due to emissions regulations, it has been removed from all petrol engine oil and alot of deisel oil as well.
It's removal is the main reason we see so many flat tapet cam failures, (yes i know you are using a roller cam)

the fuchs oil and the shell rimula range still have this aditive. It wouldn't hurt to use a bottle of crane break-in aditive in the first few sumps fulls as well .