1156/1157 vs. 7056/7528
 

Ok, so the rated voltage of the P21/5W is 12V @ 1.75A.

I'm getting around 13.1VDC at my light sockets.

What's the deal with not using 1156/1157 bulbs? The longevity of the euro bulb is supposedly 150 hours compared to 2000 with the domestic lamp with only a 4-watt increase...

If your car has the metal dividers between sockets there isn't much of a problem. If your socket only has plastic dividers then you will melt them using 1157's or 1056's.

The turn signals don't matter since they are intermittant use. Tail lights and brake lights are a different story. Before you say, "I don't ride my brakes" remember some of those long waits at traffic signals.

I can't verify the accuracy of this, but I read somewhere that if you use the US brake lamps it can irreparably harm your cruise control.

I read a lot of BS, too. The trick is picking out what's BS and what isn't.

I guess I'm in the clear - my backing plates are all metal.

I did some searching around and couldn't find where I read it. It was recently too. All I remember was that it related how the cruise control brake cutoff was grounded through the brake lamps and using the wrong bulbs (non factory spec) can damage the cruise control or render it inoperative due to some discrepency in wattage or resistance.
Might be BS but I thought it might apply to your query or at least be of interest.

[I 300SE: The wrong wattage bulbs will cause the light bulb failure circuit to malfunction and effect the the cruise control cut off signal when brakes are applied . [/I]

I guess I'll have to take a look at the wiring diagrams...