RXMacSeven

My high mileage car, 84 GSL, has finally had to be parked. Yes, I have reseached old threads but would like opinions / experiences as to what, & more so why, all of a sudden, oil in the coolant happened. One old thread said this was near impossible but it is there. It didn't overheat but I did smell coolant inside at one point. Treated with 1/2 bottle of Bars-leak(I read the CRC thread after) & smell went away. 3 weeks later smell returned so put in the other 1/2. Ran for a day & started to notice temp gauge past centre (new territory) so checked rad cap. Coolant was down & oil present. No smoke out back. Overflow well oiled as well.
I know it has to come out but would like to know what to expect. Coolant seal?? Cracked casing??? It only has 200,000+ kms.......