The use of heavier oil to increase cranking compression and gain faster starting is normally self defeating. The heavier oil slows cranking speed more than it seals
leaky piston rings and the slower cranking defeats the build up of compression.
I suggest the use of good quality 15w/40 diesel
oil such as Shell Rotella T. This will provide excellent lubrication across a broad temperature range and also allow the starter motor
to spin the engine at its maximum cranking speed.
Also, as Shakey Doug mentioned, when the engine is cool you should employ the "cold start" function which is built into the engine. To engage the cold start function you place the throttle at 1/2 or perhaps slightly more and then pull the stop cable all the way out and then push the stop cable all the way back in. This sets the fuel
rack in "cold start" position. Cold start position means the fuel pump
injects slightly more fuel
than even at full throttle, and the injection timing is slightly retarded so that injection occurs closer to TDC where the compression and internal heat is greatest. As soon as the engine starts and the RPM
starts to climb, the governor pulls the fuel rack back out of "cold start" and you just throttle back to your normal operating speed.
Hopefully this will help with the sluggish starting.