Yep, light brown is what you're looking for. Check out this O2 sensor. [link]. Genuine Bosh sensor and no gauge to buy and mount. It sends the data to your smart phone and displays a virtual gauge. Still on sale for $150.00 (100 bucks off). 

Not really. Think of the carb oil as a shock absorber.. its even called a “damper”.  Just like a shock absorber has no impact on ride height, its the spring versus vacuum that control the piston lift not the oil.  The weight of your carb oil will control the “stability” of the opening speed.  

If you look down the throat as someone stomps on the pedal you will be surprised at the speed the piston snaps open.  At a steady speed the oil will have no effect.  Sudden applications of throttle cause the piston to flutter.  

Alex, how about some dyno pics that show the impact of carb oil on output...

My experience has been that the type of oil makes no noticeable difference to how the car runs during normal driving.  So I just use engine oil (10W60).  Its certainly obvious when I’ve stuffed up and forgotten to put any oil in.  Makes the engine almost impossible to start.... but will run okay if i do get it started.  

Cheers, Ian

Thanks for confirming my thoughts specialist ! Much appreciated ! Cheers, Darren.

Thanks Michael , I’m fairly good at reading a spark plu ( I think ). Certainly not as good as using an O2 sensor,but good enough I think ( those are quite expensive- I got along fine on my Volvo [ with twin Strombergs] just by looking at the plugs. I recall a light, almost powdery brown on the plugs when in tune ? Does that sound about right ? Thanks Michael.

PS- those O2 sensors are quite expensive....

As you experiment w/ different oils you'll need some way to compare and validate your findings. Using the "seat of your pants method" isn't always accurate. Unless you have a wideband O2 sensor, your best bet is to read the spark plugs. Having a new O2 meter and comparing the plug method with the meter, I'm amazed how accurate the plug method is. I mentioned in another post that one of my chokes had stuck in the open position by a mere 1/4" although I never noticed any difference in performance—at idle, WOT or anywhere in-between. The O2 meter caught it by showing a slightly rich condition throughout the RPM range. But when I looked at my normally tan-brown plugs I was surprised at how sooty they were. 

It took we a while to understand the relationship between the carb's mixture adjustment, needle profile and the dashpot oil and spring. There is a direct relationship of course, but it's rather discreet. That's part of the fun.

Take this to the ludicrous degree to understand it. If you had NO carb oil ( very thin!) the piston would fly up, the needle would reach minimum diameter and you would think maximum richness. Nope, the engine would die. Leaned out.

Why? Airflow is not just controlled by the throttle/butterfly plates. The oil damper system controls not just movement of the needle, but also the "variable choke area" and thus airflow. This is the constant depression (vacuum) method of SUs.

//sucarb.co.uk/technical-carburetters-introduction

Thank you all for your input. It is great to get so much feedback. And I especially like the explanation about each carb being different ( different spring etc - thanks ). I’m sorry I’m just replying now, as I was under the weather all freakin week. I’m fine now ( that goodness - couldn’t get enough sleep but I think it’s because I added omega-3 to my diet ...I found this on the net, but will confirm with my doctor...I was sleeping whole days away....couldn’t get awake...sorry to get off topic :) This is great news guys. I guess I’ll send a message to the seller and see what he uses...he has 2 minis himself. But experimenting sounds like a good idea. Malcolm and Alex, it never occurred to me that a thinker oil would slow the piston down, and vice - versa with a thinner oil. Would it be correct to say that a thinner oil will cause a richer mixture ? That’s what my “logic” is telling me ? Does that sound right ? Thanks a bunch guys....I think I had better order an S U manual ! All the best friends, Darren.

100% correct. If you want more carb piston lift use a thinner oil.

Different engine like different oils based on their state of tune and modification.

I've spent *some* time on the rolling road swapping oils in Gertie's carb, alongside trying different plugs, etc etc.

+1 to what Doug said about trying different oils. There being so many variables in tuning and condition from one engine to another, and the carbs themselves, no oil is right for all applications. Since SU carbs are tunable by changing and adjusting needles, or by changing the dashpot spring, it is logical that the damper oil type and weight would also be a tuning variable. (Nobody mentioned motorcycle fork oil!)
Once you determine which oil works best, LABEL IT! I got mine to where I and the engine were happy, then missed out on a Mini driving season due to illness and now don't know which was best. That and, due to low mileage driven, the oil has not needed topping up. So, I may have to re-experiment.
PS: When I bought a new carb, it came with both the red and blue dashpot springs inside, which wasn't discovered until I took the dashpot apart trying to figure out why tuning was such a problem.

As stated above, each person you ask will have a different opinion about which weight oil is best.  Don't feel you have to go by anyone's dictates.  Try different oils, feel the difference, and decide which you want to use.

If you do want to use 20W oil, it should be available from our host following the link Harvey posted above.  You can also order it from Burlen (SU) in the U.K.  However, a ready source for 20W is the local hardware store.  Look for 3-in-1 oil in the blue/white bottle.  That's straight 20W.  3-in-1 in the black/white bottle is 30W.  The 3-in-1 oil bottles are a compact and convenient way to carry extra dashpot oil in the boot.

You should definitely use a dashpot oil

The factory used Duckhams 20w - 50. That being said if you ask 10 people you would probably get 10 different answers ranging from sewing oil to 50 weight.

https://www.minimania.com/part/GGL9035/Classic-Mini-Su-Dashpot-Oil-125ml

While I was net surfing for pictures of silver Minis yesterday I came across a rather amazing thread on dashpot oil. It became pretty heated between two gents...one claiming that using “proper “ S U dashpot oil will always give the best results on a carburetored car, and anouther who deeply believed that 20w-50 motor oil was the best. Of the others who chimed in , most seemed to agree that 20w-50 was the way to go , and one lone chap saying to use a 20 weight only was the best answer. I’m not sure if I could find a single weight 20 oil these days, but it got me wondering what forum members with carbed cars use ? Anyone care to share what they use ? I ask because I will soon need to know. My old Volvo had twin Stromberg carbs , and IIRC I got along fine with 10W-30 motor oils. So I understand the operating principals of these variable venturi carbs, but wish to start fresh with an opinion poll on dashpot oil for S U’s. How about it friends ? Step up and help me decide ? Thanks all, Darren.