I'm assuming that the 3.4 watt bulb specified is sized to closely match the resistance of the gauge sender or 44 ohms as per the FSM. At 12 volts, 3.4 watts is about 42 ohms resistance. A 5 watt bulb is more like 28 ohms. Not sure whether that difference is significant or not, but you could always add a resistor in-line to make up the difference and limit the current in that test circuit.

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
4Crawler OffRoad - 4x4Wire discount code