Intro:
If you haven't noticed by now, SUs do need periodic, if simple, maintenance. Among the checks required to ensure top performance is the inspection of the damper oil levels.
The damper (the black plastic screwy thing on the tops of your SU domes) acts somewhat like a shock. The damper/oil combination helps to regulate the piston movement under driving conditions. With proper oil levels the dampers will be completely immersed in oil. You are here now because those oil levels are not correct or you'd like to experiment.
Symptoms:
The main symptom of improper damper oil level is an off-idle/low-speed stumble in acceleration. This happens because, with oil levels that are too low, the distance between the oil level and the damper allow the piston to flutter up into the bore. This premature lifting of the piston disrupts the metered air and fuel flow into the engine and gives you a stumbling/lean acceleration. Upper rpms are not affected because the piston has raised enough to cover the damper in the remaining oil.
The necessary inspection period for damper oil levels depends on your driving style. If you do mostly freeway commuting you can lengthen the inspection interval. If you have a heavy right foot and/or do a lot of driving in traffic, you'll have to inspect the oil levels more frequently.
The Datsun Factory Manual specifies an approximate inspection period of 3,000 miles, or every three months. I would say this is appropriate for the average driver in average conditions, and it also coincides with a typical Datsun owner's engine oil change. If, however, you're on the harder side of use, I'd recommend a monthly check of oil levels.
The easiest way to check the oil levels is to first remove your air filters. Raise each piston up into the bore with your finger. You should immediately feel resistance and it should start at the same point on each carb. If you feel any "free play" as you raise the piston (before the oil is raised in contact with the damper), then the level of oil in that carb is low.
Damper Oils:
(Also see Advanced Techniques, below)
There are as many different suggestions for what type of oil to use in your SU dampers as there are oils. Datsun recommends 20WT or 10W30 engine oil. I'm currently using 20wt motorcycle shock oil (not having found any 20WT engine oil). I used to use ATF (which is around 10wt) but I found that it was too light and didn't provide enough dampening. This was only evident after installing my air/fuel ratio gauge. This caused my air/fuel ratio to go lean whenever I opened the throttle. The 20wt oil has cured that.
However, which oil to use is also dependant on your climate and weather conditions. I live in southern California, which is pretty mild year-round. We don't get really cold temps, nor to we get extremely hot temps for very long. In a colder climate, you may want to stick with the 10WT automatic tranny fluid. Hotter climates will probably get proper performance from 20WT, or a mixture of tranny fluid and 30WT oil (or 10W30, etc.). If your climate varies according to the season, you'll have to change damper oil to suit your conditions, just as you do for engine oil.
Datsun specs list the total volume of oil (per damper) at 3 cc's. I have not verified exactly where this will put the oil level, though you are free to try it out and let me know. If you don't have a way to accurately measure that amount of oil, you will just have to be aware of three concerns:
You will have to remove the SU damper, dome, and piston spring from the carb body. You should only work on one carb body at a time so you won't mix up parts between carbs. Always keep each SU dome with its corresponding piston on the same carb body.
After you have removed the piston from the carb body, pour out any remaining old oil. Dispose of the oil properly according to your state's enviromental laws. Slowly pour the oil into the piston until it reaches approximately 2/3rds of an inch from the lip. Test-fit the damper all the way into the piston. I fill the oil until I can put the damper in all the way without spilling any oil. You'll get the idea when you try it.
Damper oil can be used to adjust or tune the lifting of the pistons to suit a modified engine. Since I installed my Air/Fuel gauge, I have been able to see that the ATF fluid I'd been using was far too light a weight oil for my application.
For example, under cruise conditions I was getting a fairly rich reading on the air/fuel gauge. Any increase in throttle gave me a lean condition immediately. This was because the oil in the dampers was not heavy enough to keep the piston from opening rapidly. The sudden increase in vacuum from the motor draws up the SU piston (which allows more air into the motor). With a bigger hole from which to draw air through, vacuum over the jet bridge does not rise as quickly to pull more fuel into the stream (despite more air getting into the engine = lean air/fuel ratio). The low vacuum doesn't allow enough fuel to be sucked from the jet to compliment the increased air flow.
3/15/99
I've purchased some 15wt motorcycle shock oil. Replacing the ATF with the 15wt oil has done wonders for the air/fuel readings. Now even with heavy throttle application the fuel mixture does not go lean. Obviously the heavier oil is not allowing the piston to rise as fast as it was before. The seat-of-the-pants differences are not dramatic, but at speed (say, on the freeway, and at higher revs) you can feel a little more urgency out of the car. Whatever the other benefits, the mixture is not going off-the-gauge lean as it was before, and this is good for my peace of mind. Without the air/fuel gauge, however, I wouldn't have known I had a problem as it drives about the same.
4/27/99
The 20wt oil (again, using a bottle of motorcycle shock oil) has gone into the SUs. I haven't noticed much change at all. What change I have noticed has come courtesy of the air/fuel gauge. I was getting a bit of a lean-mix spike with throttle application on the freeway. This has lessened, but is actually still there to some extent (it occurs more often before the motor has fully come up to temp). Once the motor has come up to temp (long after the temp gauge stabilizes), the 20wt oil seems better suited to prevent the lean condition mentioned above. However, with summer coming, I may have to step it up again. We'll see.
11/21/99
Summer came and went. As it is November, and getting slightly chilly 2 out of 7 days here in SoCal, I went back to the 15WT oil. However, on my drive to the SoCal UFO swap meet during the weekend I noticed my air/fuel gauge going lean under acceleration/load. This was most noticable as I went up a few grades on my way. It wasn't terribly warm that morning, so it would seem that 15WT is too light for a SoCal car (at least with my motor). Back to 20WT, where I'll stay.
6/15/01
I've gotten into the routine of going with 15wt oil in the SoCal "winter" months, and 20wt oil in the summer months. When I change my coolant thermostat from a 195' winter to the 160' summer, I also change damper oils. In my climate that's all that's needed to be messed with.