Hello.
The preparation time for microscopy for an ISO VG 680 grade oil will take time (I would estimate 1 hour for 100 ml oil).
The counting (ISO 4407) is also time consuming -I would estimate approx. at least 15-30 minutes for an experienced analyst.
I would recommend to look into the LaserNet Fines from Spectro. You would need to dilute the oil with some clean solvent due to the high viscosity.