In the IEC world it is the same oil, usually mineral oil to IEC 296 or BS 148. In large system transformers and generator transformers it is rare to see Midel and silicone because of the large volumes involved. Not impossible if you have the money to spend, but very unusual.
Yes, the BDV in kV/mm is the same, but obviously the higher voltage transformer has greater internal clearances around live parts.
System owners with numerous small distribution transformers often allow their oil condition to deteriorate to a state far poorer than would be allowed by an operator of a large transformer. This is because in the former case the maintenance costs are relatively high, asset numbers are large, the individual asset value is relatively low and the business impact of losing a transformer is relatively low. For a large system or generator transformer the reverse of those four statements is normally true and the maintenance is much better.
Oil for all the voltage level is same. Only difference is in its BDV and PPM level. For higher voltage level e.g. 200 kV system, desirable BDV is 70 kV and PPM is 100, whereas for lower voltage level e.g. 33 kV, 55 kV and 20 PPM might be acceptable.
Today transformer oil all over the world is going by two standards IEC or ASTM.Once upon a time, BS 148 from UK was followed by most of the power engineers.This standard was first published in 1923(those who want to read it refer to first or second edition of J&P Transformer book published in 1925)After going under eight revisions,in 2009 it was withdrawn and replaced by BS EN60296 almost lining up with IEC.Now BS148 ed9.0-2009 covers specification for Reclaimed mineral insulating oil for transformers and switch gear and not new unused oil.
IEC on oil first came out in 1969 as IEC 296 ed1.0. After three editions,current standard is IEC60296ed4.0-2012. As per this standard there is only one grade of oil for all types of transformers.(pour point-40C) There is a low temperature switchgear oil with pour point -60C for switchgears. Oil can also be of three types depending on the amount of oxidation inhibitors added in to it. U grade (uninhibited when inhibitor <0.01%) Tgrade (trace inhibited with inhibitor content <0.08%) I grade(inhibited with inhibitor content of 0.08-0.4 %) There is also super grade oil recommended for high temperature service or for extended life. This is a super refined oil with minimum sulphur content but always inhibited(I grade) with better oxidation stability characteristics.This is preferred by some users for EHV Transformers,HV DC Transformers,Arc Furnace Transformers etc.
ASTM D 3487-2008 ed4.0 (first published in 1976)covers two grades-Type1.0 trace inhibited and Type 2 inhibited.It is significant that as per ASTM today there is only inhibited type oil.2008 edition also changed anniline point so that standard allows both naphthenic and paraffinic oils.Earlier editions was allowing only naphthenic type.
