Jeep in line sixes are notorious for making lifter noise more then other motors. Because the lifters are right against the block instead of burried inside like a v8, you hear them more. When things get cold they shrink, so every thing is a little loose inside your motor.
The loose clearences and cold oil mean it may take a few minutes for everything to be lubricated and up to operating temp.
It is probobly nothing to worry about and I deffinently would not recomend a oil thinner then 10w30 in a jeep inline with it's old castings, loose clearences, and non roller old school camshaft/lifters.