Yes a HT loss costs oil, but the point is that normally you shouldn't loose them, after some practice it's the case.
For WW I don't have problems to 5 star high evolved bases, the key is to have maxed aviation (mk6 assault fighters + maxed amelia) and with the howitzers from alliance donation (4 howies) and the 3 I can add myself to my five HT it's enough to finish a base, furthermore I spare 8 infantry untill the end to clean the rest of the map (and planes also help for this). To this you have to add the 3 generals, the mercenaries and the coalitions armies which can also help to finish a map.