I think you're being a little overly paranoid about it. The techs are factory trained and an oil change is probably the easiest thing they have to do. Give them a little credit here.
The tech probably seemed nervous because he never had someone interrogate him about his work before.
AW/Terra | ZSP | ZPP | ZCW | iDrive | CA
Mods: BC BR coilovers | Blacklines | M-tech rear | dual exhaust | M3 steering wheel | BMW SSK | ZHP shift knob | High-kick CF spoiler | NBT Retrofit
M3 front control arms | M3 rear guide rods | Manzo rear toe and camber links | Monroe rear shock mounts | Varrstoen ES1 19x9.5+20 Front, 19x11+25 rear