intercept it at perihelion this is why.
your ships relative velocity is 20 km/s at the interception and moho's relative velocity is 18 km/s do the math
V, your ships velocity
X, body's relative speed.
Z, insertion delta v.
v-x=z
20-18=2
=2 km/s of insertion
now
intercept at aphelion
16-12=4
=4 km/s of insertion.
see, now this will save you about 1 km/s of delta v.
you'd be thinking why not 2 km/s because insertion is 1 km/s more than the second way.
but still saves delta v.
also your periapsis must be adjacent to moho's periapsis.