In the post war era SU became the carburetor of choice for many English car manufacturers. Several types were developed, and used concurrently.
SU will supply specific oil for this purpose. It is essentially 20 weight engine oil. In the pre-war period various types were recommended, such as "Bicycle Oil". In the 1960's some OEM installers like Volvo advised ATF. Regardless, the OEM installer and SU designed and adopted the needle profiles according to the oil they recommended. So what happens with other oils? The answer is simple. The higher the viscosity, the more pronounced the enrichment during throttle transitions for acceleration. As well, use of a multi-grade oil will vary the enrichment in the opposite to that needed during warm-up to operating temperature. If the oil selected, be it gear, engine, transmission, sewing, or synthetic, causes no unacceptable symptoms, it is fine. Certainly nothing will be damaged.
SU specified fewer than ten piston springs for all types, and for the HS type only four are suitable. SU also supplies over 600 needle profiles for the three jet orifice diameters. Obviously the fuel needle is the most important aspect of tuning for a particular engine. As long as the springs are undamaged, and equal in length, successful tuning is possible. Early SU's, and some used into the 1950's used no springs at all.
In the SU carburetor achieving good idle mixture quality is achieved by moving the jet in relation to the needle. This determines the mixture to a lesser degree over the entire operating range. Any air which enters the intake tract without passing over the bridge under the piston (false air) produces a leaning affect, which must be compensated for by lowering the jet orifice. This may correct the idle quality, but it will upset the mixture at higher engine speeds even though the ratio between false air and venturi air is far smaller.
Worn throttle shafts and bushings also produce a variation in idle speed, as the two components rarely sit in the same relative position each time the throttle is closed. A correct idle speed may suddenly be too fast or too slow depending on this position. Also setting the idle speed may be troublesome as one slows the engine gradually only to find a sudden drop in RPM. Achieving the correct idle speed, and having it repeatable over all throttle closings may be almost impossible.
New throttle shafts fitted into newly installed bushings restores the air flow through the carb, forcing all air to pass over the bridge. As well, the angle of the shaft directly controls the idle speed without any annoying "dropouts" in RPM. The carb responds reliably and easily to adjustment, and holds that adjustment over time.