I'm fairly certain that the Merc blue hi-po lube is an SAE 75W-90 lube, which is the equivalent to ISO VG 150.
The most closely matching Bel-Ray gear lubes are:
(1)Bel-Ray SE2 Lubricant, P/N: 63700, ISO VG 150.
Synthetic General Industrial Lubricant.
A technologically innovative synthetic extra heavy-duty gear lubricant
formulated for all types of gearing under severe temperatures and
loads. Effective for plain and roller bearings. See Product Data Sheet
for name grades, temperature ranges and application instructions.
(2)Bel-Ray Synthetic Gear Oil, P/N: 66880, ISO VG 150
Synthetic General Industrial Lubricant.
Totally synthetic hydrocarbon and ester combinations exhibiting
outstanding Extreme Pressure and anti-wear properties as well as
excellent viscosity and temperature properties. This lubricant exhibits
low coefficient of friction in addition to being compatible with mineral
oils.