Date registered: Oct 2007
Vehicle: 07 SL350, 14 BMW320, 05 SLK and 4.6 MGBV8
Location: Living well in S France.
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There is no such thing as an SL500 AMG - as everyone has said, these have proliferated since the rise of ebay.
AMGs will only have a two numeric digit combination following the model designation - 73 (very few ever made, it is used only by Pagini now), 65, 63, 60, 55, 43, 36, and 32. (The latter arguably the best of them all, but it embarrassed the 55 by being too close to it in power output. As the 55 is really made for the US market, and the V6 was never destined for the US, it lost out and put back on the shelf). The only variances to these were the older 560SEL-AMG and the 'Hammer 330E which were pre Mercedes owning AMG.
Technically the only really 'true' AMG engine is the 63 using the AMG designed M156 engine, whereas the others are 'mildly' tuned Mercedes engines.
The engine in your 55 is not smaller than your S550, but you can try! Good luck on the hunt. Try and get one with a warranty - they are a pig when they go wrong!