Honda VTX1800R Owner's Manual page 187

Specifications
Fuel & Lubricants
fuel
recommendation
fuel tank capacity
engine oil capacity
engine oil
recommendation
final drive oil capacity
cooling system,
recommendation
cooling system,
capacity
178
Technical Information
unleaded gasoline, pump octane number of 86 or higher
5.28 US gal (20.0 )
after disassembly:
after draining: 3.7 US qt (3.5 )
after draining & oil filter change:
API Service Classification SG or higher except oils
labeled as energy conserving on the circular API service
label, SAE 10W-40, JASO T 903 standard MA,
Pro Honda GN4 or HP4 (without molybdenum additives)
4-stroke oil (USA & Canada) or Honda 4-stroke oil
(Canada only), or an equivalent motorcycle oil
after draining: 4.1 US oz (120 cm )
Pro Honda HP Coolant or an equivalent high quality
ethylene glycol antifreeze containing corrosion protection
inhibitors specifically recommended for use in aluminum
engines
3.4 US qt (3.2 )
including reserve
4.8 US qt (4.5 )
3.9 US qt (3.7 )