I am frequently asked what oil to use in SU dashpots. To me, the answer is clear: Use Mobil 1 15W-50; nothing else works as well. Nothing.
The earliest dashpot oil recommendation that I can find is in the Owner’s Manual for my 1960 Volvo PV 544, with B16B engine and Type H-4 carburetors. At the bottom of Page 32, it says: “Use thin engine oil (SAE 10W).”
The next recommendation that I find is for my 1965 122S with B18D engine and Type HS-6 carburetors. On Page 62, Note 10 says: “… top up with light engine oil (SAE 20 — not multigrade oil).”
Later recommendations seem to gravitate toward ATF, which I believe to be a totally inappropriate oil for SU dashpots.
In 1972, I purchased a 1-quart supply of SAE 20 oil, and I used that in both my PV-444 and my 122S, with acceptable results, until I ran out sometime about 1990. When I ran out, I couldn’t find any more SAE 20 that was not multigrade. By that time, I had junked my 122S and was running my PV-444 with B16B engine and Type H-4 carburetors. I then went looking for a suitable dashpot oil and tried several different types and viscosities.
Among the oils I tried were: ATF, Mobil 1 5W-30, Mobil 1 10W-30, Mobil 1 10W-40, and Mobil 1 15W-50. Of all those oils, Mobil 1 15W-50 is clearly the best.
Note that, at 0 deg F, the Mobil 1 15W-50 has a viscosity similar to that of a straight SAE15 oil, whereas at 212 deg F, it has a viscosity similar to that of a straight SAE50 oil. Somewhere between those two extremes of temperature, the viscosity curve of Mobil 1 15W-50 must cross the curve for straight SAE20 oil. That is to say: at some normal under-hood (carburetor) operating temperature, Mobil 1 15W-50 will have a viscosity essentially the same as straight SAE20 oil, which not coincidentally, is exactly what Volvo recommended for my 1965 122S.
Not only that, but because the Mobil 1 15W-50 is a multi-grade oil, its viscosity is much more stable than the SAE20, so its viscosity tends to stay near the optimal value in both hot and cold climates, summer and winter.
Although I have not personally used the “Special” SU damper oil sold by several different retail outlets, I have consulted with several people who have. Those who, on my advice, switched from that rather expensive special SU damper oil to the Mobil 1 15W-50 that I recommend have been very glad that they did.
ATF is a lousy product for this purpose. It’s not thick enough. You need a higher viscosity oil to provide greater resistance to upward movement of the piston and needle on acceleration. That causes a bit of “choking” action on acceleration which reduces the hesitation often noticed on acceleration with cars that have SUs. ATF also seems to disappear from the dashpots at a rapid rate, necessitating frequent replenishment. Those who have used the special SU oil have told me that it also requires frequent topping up.
Also, if the dashpot oil is too thin, there will be insufficient damping of the piston on acceleration, and the piston will rise too far too soon, which decreases the vacuum at the bridge (that is, at the jet and needle) which decreases the fuel coming through the jet, resulting in a lean mixture and hesitation on acceleration.
SUs really do benefit from using a somewhat thicker (more viscous) dashpot oil than ATF or similar oils.
I highly recommend Mobil 1 15W-50 for use in dashpots. It provides the damping needed, and it doesn’t seem to disappear from the dashpots as quickly as lighter oils (especially ATF) do. So, much less topping up is required.
By using Mobil 1 15W-50, you can adjust your jets a bit leaner and still avoid hesitation on sudden acceleration. Whereas, with ATF, you will have to run a richer mixture at all times to keep the hesitation down to an acceptable level.
One of my carb rebuild customers (James Schoster, who I think missed his proper calling of Mechanical Engineer) took some videos comparing the viscosities of Mobil 1 15W-50 and the special SU damper oil. Unfortunately, I cannot post videos on this blog, so I have to settle for a couple of still shots, captured from my computer screen, which still give a pretty good idea of how different the two viscosities are.
First, the “special” SU damper oil:
Notice how the special SU oil runs through the syringe in a steady stream. Timing the video with my stopwatch indicates that the oil level drops from 4 ml to 2 ml in about 8 seconds, or roughly 0.25 ml/second.
Notice how the oil is falling as discrete drops. Visually, it appears to be roughly 2 drops per second. My stopwatch said it takes 10 seconds for the level to fall from 4 ml to 3 ml. That’s a rate of 0.1 ml/second.
Clearly, the viscosity of the Mobil 1 is considerably greater than that of the special SU oil.
My customer (James Schoster) switched from the special SU oil to Mobil 1 15W-50 and was very pleased with the results. Several others have done the same and all of them have reported improved performance (less hesitation) with Mobil 1 15W-50.