From the 2006 specs on the 1GG-Fe
SOURCE
8. Crankshaft Bearing and Crankshaft Bearing Cap
The crankshaft main bearings are made of aluminum alloy.
Similar to the connecting rod bearings, the lining surface of the crankshaft bearings is micro-grooved to
realize an optimal amount of oil clearance. As a result, cold-engine cranking performance is improved and
engine vibration is reduced.
The upper main bearing has an oil groove around its inside circumference.
The crankshaft bearing caps are tightened using 4 plastic-region tightening bolts for each journal. In
addition, each cap is tightened laterally to improve its reliability.
I do not know if this is new for 2006 or not.