Most of the labs I have used all use hexane as a solvent. The hexane (regardless of what grade ie technical or analytical) is filtered through a 0.8 micron filter prior to use. I believe that acetone is sometimes used particularly when the sample has water. The key thing is that any solvent used should be put through the same cleaning process - it is possible that if the solvent used is not filtered which can cause a variation in the reading.
I have used 3 different labs for ISO via Patch testing from time to time, the most variation I have come across is by one ISO code for different operators.
The comparator labs use could possibly cause a variation in the reading - for example both PALL & Hydac have comparators out in the market - I have seen slight differences in the interpretation depending on which comparator is used.
Personally, I am interested in oxidation by products as well as insolubles and wear contamination as I believe all these help towards making a better judgement on the oil and machine condition.