I suggest rto perform oil particle analysis folowed by ferography or/and ICP it seemes a contamination or non compatible materials inside the trnasformer to oil.
If you have a similar transformer it will help to examine same test.
TC in US differ from Europe. In us are on the low side and in Eurpoe in the high. DGA is not usefull becouse is TCG mainly depend on number of operation. We now investigate other techniques like metals, particles.
Sorry for the delay
We are measure dependence of gas in transformer oil solubolity mainly for the headspace application. The Henry law is obeyed, in the way that increasing the pressure will lower the gas concentration in gas phase. This is not appeear in the standart (ASTM) deal with this...