5w20 is a high temperature oil suited for hydro-dynamic "bearings" and for "smoke of oil" to lube the piston in an engine.
75W90, manual transmission oil, is suited for EHD (elasto-hydro-dynamic) best for Hertzian efforts found in ball bearings, involute spur/angled gears, chains and any other sort of Hertzian effort (where the area of contact is very small, a point, or a line)
Chains are best maintained and lubed being submerged permanently in oil like in the timing chain in car. Also the best chains do not have rollers, the plates have an involute profile that match on involute spur gears.
But for a bicycle we have to deal with a roller chain (we want more than one gear, so we need to be able to shift the chain on different cogs) and not submergerged in oil.. it's not posible, it would complicate the whole thing and would add some 2kg.