There was a basic difference between U.S. fuel and the fuel in England, at least at first. Our (U.S.A.) fuels were 4% aromatics and British fuels were 20% aromatics. So, they could never replicate the issue on the test stand until someone brought over some British fuel. That took about 9 months to happen. Instant replication on the test stand. Do NOT get me wrong, there was nothing wrong with British fuel at all. It was just different from what was used at the factory for tuning purposes, and the engines were misjetted for it from the factory. Once the issue was known, the issue went away rapidly.
This problem was long gone by the time the P-38s showed up in europe in ernest (late 1943/early 1944).
In 1940/41 the US was using the low aromatic fuel (some sources say less than 2%) while the British used the high aromatic fuel, not LESS THAN 20% (could be more).
The American fuel was about 100/100. You can't make 100/125 or 100/130 fuel without either using a large amount of aromatics or prohibitivly expensive base stock and a lot more lead than they wanted to use in a service fuel.
The British fuel in the BoB was around 100/115-120. It varied from batch to batch and they had no way to measure the higher number (rich response) until the Performance number scale was developed. Which was done soon after the BoB ended. We are still 3 years from P-38s showing up in Europe for escort duty (aside from a few missions in 1942 before all available P-38s were sent to North Africa).
The US quickly specced out a 100/125 fuel and a number of engines were rated on it. How much was made and/or how wide a distribution in the US it got I don't know.
The British jumped right for 100/130 fuel once they could measure the rich response using the PN scale. However the Americans and British decided well before Pearl Harbor that having a joint fuel specification would be a good thing so the US adopted the 100/130 fuel (we are still over 2 years from the P-38 showing up in Europe for escort duty).
Then the production problems came in. The original 100/130 joint fuel specification called for not more than 3.0cc of lead per gallon. A large increase in hundreds (if not thousands ) of tons of 100/130 fuel could be made if the standard was relaxed to 3.6cc per gallon by using slightly lower grade base stocks and/or less of some of the rare aromatics. this new specification was either late 1941 or early 1942. In late 1942 they were figuring how to increase production of 100/130 even more. It was decided to go to a max of 4.6cc of lead per gallon (not all fuel blends required the max amount of lead) and also to increase the max allowable percentages of certain heavy aromatics ( there are over a dozen aromatic compounds, not all suitable for motor fuel). Large increases tonnage of 100/130 fuel were made from similar quantities of base stock without having to devote thousands of tons of high grade steel to refinery production.
However some of the new fuel blends (and there were a large number of individual blends that met the standard specification) required different spark plugs or more frequent spark plug changes. The problem of fuel puddling in the intake manifolds was suspected early (the heavy aromatics seperating out) and this was in early 1943, well before the P-38s showed up in Europe. Allison started work on the intake manifold with the turbulator in the spring of 1943. It wasn't ready until the end of 1943 at which point it was used on ALL Allisons regardless of the type of airplane they going in and replacement manifolds were shipped out to be fitted to engines in the field or as they came in for overhaul?
This problem with the later fuels in 1943/44 had nothing to do with the difference in fuel between the British and the Americans in 1940-41. the 1943/44 fuel was at least 3
changes in specification from the fuels in use in 1940/early 1941.
Running the P-38s at low rpm and high boost for cruising kept the engine a bit hotter, the turbos hotter and the intercooler hotter which helped prevent the fuel puddling but the army didn't want to run them that way, they wanted to use high engine rpm and low boost, thinking that this allowed for quicker time to max power from cruising settings.
The later P-38s got a generator on each engine which gave them the extra power to run electric cabin heaters. The ones up to the beginning of 1944 only had one generator and since so much of the airplane was electric (like the props) overloading and killing the generator was a mission kill if not a loss of the aircraft.