I just recently purchased a 2001 CL with 47k on the clock. It was certified, but I did not buy it with the certification. I also have an '02 Audi S4 and '00 Passat GLX (2.8 litre 30V engine). I use nothing but M1 0w-40 in the german cars, but they were both purchased new. I know that Honda's like thinner oil (mainly for fuel economy reasons, IMO) but I am a confirmed believer in sythntic oil for any vehicle that you care about and have been using it for the past 10 years at least.

M1 is good quality and relatively cheap (for synth oils). If you want the absolute best, Motul and Redline, along with Amsoil are great; but also cost top$. Royal Purple is likely very good, but the guys on AudiWorld (my primary home) have not gotten as good of results as with the Motul, Amsoil, etc. They have a lot of actual oil analyses done by Blackstone Labs to back up the talk, not just blowing smoke out their a$$.

For the CL, I decide to go with Valvoline full Synthetic (I think they call it SynPower). Castrol Syntech is also OK, IMO. Here's the deal on what I did: bought the car and although it had just had an oil change (with who knows what and it was also at least 1/2 qt overfilled - oil was like 3/4" above the top hole in the dipstick) I changed the oil and used a Purolator PureOne filter. The oil I used was NAPA brand synthetic - it is made by Valvoline and just repackaged under the NAPA name, and it cost $2.99 on special per quart.
The filter on there was some cheap Fram POS (I openly admit I HATE Allied signal and anything they produce, so realize I have bias against them even though they supposedly make the Honda brand oil filters).

I change oil every 3k, and only use synth. I know this is likely wasting $, but I like going the extra mile on my vehicles. If you want to go long drain intervals, I would recommend the M1 and would go with the 0w-30 weight if you live in the North (cold weather), or the 0w-40 when its summer.

The important thing to me is the ACEA ratings (European oil standards like the SAE "SL" ratings for the US) and the A3/B3 ones since they are for extended drain intervals and engine wear protection. The A1/B1 specs are for fuel economy only. The tiny (and I mean really tiny) filters on Honda products puzzle me, but the Japanese don't use any more of anything than is absolutely necessary (the Audi & VW use a filter that holds 1 qt of oil and the oil pans hold 7 qts and 6.5 qts respectively). The small filter is why I will change every 3k on the Acura, no matter what oil I end up using in the long run.

Main benefits to synth are superior lubrication, superior adhesion to engine parts (better for start-up wear), and also superior high temp stability (this also results in less oil volatility or boil-off). Some oil loss is due to the lighter fractions boiling off and getting sucked through the PCV valve into the cylinders and burned; synth greatly reduces this type of loss. Also the synth has much better viscosity stability versus dino-crap.

If your engine is in good shape (no oil burning or plug fouling) I would def. go synth if you don't mind spending a little extra (remember that the extra $ it costs for synth over dino oil is less than 1 tank of gas). Also, I highly recommend to change plugs before 100k miles - mine are getting changed within the month (at like 48k). They can rust-weld to the aluminum heads after 100k and you might damage the threads removing them and you are then screwed.

Good luck with your CL and hopefully both of us will be well over 100k with no problems (as long as the tranny doesn't blow up - be sure to change that fluid often!!!). Register on the Acura website and they can tell you if your tranny had the recall done and it also states the warranty extension on the tranny (7 yrs/100k miles).