Check the dipstick for contamination of the sump oil, but my bet would be on your oil cooler being fubarred, & as for the smoke from the filler neck, your breathers need cleaning as it is indicative of excess crankcase pressure. Check your plugs too, they may be sitting in a bath of oil if the extra pressure has done for a cam gasket

As for cleaning the cooling system of gunk, some people use a biological washing powder mix, others use propriatory brands of system cleaner available from the likes of Halfrauds, some have even used dishwasher tablets! It is a fairly long job to remove the gunk from the system & may need doing several times before it is back to being reasonably clean again. 

http://www.omegaowners.com/forum/YaBB.pl?num=1285337586

Try this I think it has all the info in one thread following my recent oil cooler change.

marcbucks wrote on 11. Nov 2010 at 15:15:


Once the cooler is fixed....

Drain as much as you can. Refill and add a "low suds biological detergent" Run until hot + 30 minutes with car heater on max. Allow to cool but before actualy cold, drain. Repeat about 6 times or until all signs of oil have gone. Drain and fill with clean water. Run for 2 hours. Check water  if any signs of oil .. repeat the whole procedure.

If at any time during the early runs the engine shows any signs of overheating, switch off, allow to cool, drain.. then start again ..

It takes forever because the water pipes in the rad and heater matrix are very small and block easily. Repeated flushing will eventually clear them.... hopefully