John Anderson's Introduction to Flight has the equations for Range and Endurance, both for reciprocating propeller engines, and turbo jets/fans.
For a normally aspirated piston prop the Breguet formula shows range is maximised with (i) highest propeller efficiency (ii) lowest SFC (iii) maximum L/D (not 1.32Vy which is Mmr for jets) and highest ratio of fuel weight. This speed for L/D max is the tangent to the Power required curve and approximately Vy. In a piston prop, no wind condition, maximum range is independent of altitude, although rho is a factor for maximum endurance.
To achieve lowest SFC jets like cold air, and the calculus uses the Thrust required curve, hence the tangent being 1.32Vy. This speed is where (CL square root)/Cd is maximum, or where zero lift drag is three times lift dependant drag. SFC improves massively the lower the turbine inlet temperature, and rho or density figures in the Jet Breguet formula as inversely proportional to Range. In theory if the wing could fly and not encounter transonic drag, or low speed buffet, the highest possible altitude is optimum in a jet.
The maths is quite elegant, but cutting through it, it is nice to see that a Super Cub optimum altitude for either range or endurance is MSA, or away from built up areas and TV masts, around 500 feet AGL.
In the 95 HP SC this equates to 45-50% economy cruise at around 75-80mph. This assumes propeller efficiency for the fixed pitch prop is close to optimum at around 75-85 mph. Arguably 80-85 mph is more speed stable (L/D Max is speed neutral), and smooth air altitude (assuming no wind) would be ideal. Climbing to full throttle altitude at this IAS might improve SFC slightly but not to overcome the fuel used to climb. The L18C has 35 usg capacity, so at 45-50% power this is close to 9 hours with VFR reserves, or around 700 sm.