The SU carb people have a product they sell for their carbs. BMC/Leyland recommended that product at one time in the driver's manuals but at other times recommended engine oil. Over the past 50 or 60 years people have used a lot of different oils in the damper for specific purposes or because it was what they had. In my opinion it isn't really critical and if you're trying to make significant changes to your engine by changing from 20 weight to 30 weight oil in the damper you are probably looking in the wrong place. 

On the subject of SU Carb Co. damper/dashpot oil, I looked at the website again. Damper Oil SAE Grade 20    BDR125SINGLE says it's "For use in Carburettors which have a Damper fitted". Dashpot Oil Light    BDP125S says it's "For use where no Damper is fitted". These comments are at the bottom of the specific page under Details. 

Kelley

I'm pretty sure that the stuff that Castrol distributed as "SU Damper Oil" was marked as 30wt...

However, I don't think its really that critical.  Have a look at how fast the piston moves when you snap teh throttle open.  Doesn't matter what sort of oil you use the results are almost instantaneous...

Cheers, Ian

The magic oil to use in an SU dashpot is something that enthusiasts have argued about for decades. 20WT works fine in cool and moderate climates. I find 30 and 40 weight works better in an Arizona summer. In Florida, I found that power steering fluid worked fine year-round.

Main take-home lesson is to not worry about this too hard. Any light-weight oil will do fairly well.

Kelly -  Your comment that the part numbers "are almost the same" indicates to "me" that they are not the same, therefore have different uses.  That various oils can be used to damp the carb is not in dispute and using only an "eyeball" comparison look at the two oils indicates one is lighter in color (possibly being lighter in weight) than the other - 20wt oil is recommended by S.U.  What I'm looking for is for someone who has used "something" else (meaning another 20wt oil) and whether it was satisfactory or not.  My thinking is that if "anything" is satisfactory for damping, why did the S.U. Carb people even bother to "recommend" a 20 weight oil instead of a standard 10 or 30 weight ND motor oil?

I use 20W/50 engine oil in my SU's (same as they used at the factory service department) and have used ATF on one car that seemed to run better on it (maybe needed a needle change).

I use Guinness Extra Stout during the summer, and thinner Hefeweizen in winter.  Make sure there is ample supply of both under the rear seat.

The maltier Porter Ales will gunk up the plunger slightly but at least are very good to the seals.

For cooling the thinner, hoppy Pilsners also will keep the water pump lubricated and have a higher boiling point than Prestone.  After topping off run the engine for several minutes with the cap off to let C02 vent and prevent foaming.

There is no difference in the products that Burlen (SU Carbs) is selling. They practically have the same part number. They are both damper oil to be used in the SU carb. 

What follows is a pet peeve of mine. Damping is a word related to oscillation and vibration. Dampening is a word related to moisture.

Kelley

"True or not?" Not! The damper is to retard the upward movement of the piston & needle on acceleration only ... to promote rich mixture. Therefore the mixture might be lean (or rich) on acceleration if the damper oil is the wrong viscosity. At steady rpm the piston height is determined by the piston spring and the airflow demanded by the motor at that particular rpm. I suppose if you were constantly accelerating and decelerating as in a race car a too lean oil might cause over heating but your race car would also running S***ty because of lean out (on acceleration).

The S.U. Carb people list "dashpot" oil and "dampening" oil, with the latter listed as to be used to dampen the movement of the carb., the previous one used for carb's that  have no dampener.  In talking to a couple of suppliers I was told there is no difference and both can be used to dampen the movement within the Carb.  Anyone know?  Next is a summary of various articles.  Using ATF oil (about 10wt) is best used in cooler climates such as the Midwest and east coast of the U.S. and 20Wt oil (as recommended by S.U.) should be used in warmer and hot climates such as the Southwest U.S. where temp exceed 95 degrees.  Using 10wt oil in hot areas results in a leaner carb mixture and could result in higher engine temp's.  True or not? Next - 3 and 1 oil is a 20wt ND oil ($4 US for 8 oz) and has been satisfactorily substituted for 20wt carb oil (which in my part of the world is selling for $13 (US) for 4 oz, plus $10 (US) shipping).  Also some M/C's use a 20wt "fork" dampening oil that is reasonably priced. Any thoughts?  I'm not overly frugal but can't see throwing money away.  I have three vehicles that have S.U. carb's and top off about twice a year.  DanD