jeff,

yes it's possible. but there is some risk involved. the procedure is as follows:

1) drain 1L of old oil
2) add 1L of kerosene
3) run engine at low speed (1000rpm) for 30min - 1hour until wala nang black smoke

4) let engine "soak" overnight
5) run engine again until wala nang black smoke
6) drain all of old oil+kerosene, replace with 20W50 low-grade oil, replace filter also

7) run car for an hour or so, check if oil is turning black (kasi merong mga naiwan na crap inside the engine)

8) if turns black, replace oil AGAIN

the problem with this method is, kung high-mileage na yung car, there might be parts that are held up only by carbon deposits (e.g. damaged valve seat, the carbon deposits act to "kalso" the damaged valve seat).

using kerosene is quite aggressive cleaning, matatanggal lahat ng carbon deposits. so it might remove the "kalso" also baka merong carbon deposits na maiwan sa loob ng oil channels, which will later cause loss of oil pressure and subsequent engine failure.

kaya the safest bet is, just add 1L of ATF to your next oil change. OR, use full synth oil (same effect, actually safer, pero mas mahal...)