Hello KR,
1994 saw the introduction of the 'interim' engines and these were fitted with a crankshaft driven oil pump. As such you have a much more efficient pump compared to the camshaft driven variety and as such the oil spec is a little different. Both 10W50 and 15W50 oils are the recommended ones for the 4.2, both in a semi synthetic such as those offered by Penrite. The reason for a semi synthetic is the operating temperature of the engine places greater stress on a straight mineral oil, so for better oil longevity and engine protection, semi synthetic is the best choice.
Ron.