Keep in mind though that you can't just willy-nilly switch oils unless you're sure of what you're putting in there. Say you have Havoline brand, which is paraffin based, and mix it with Penzoil, which is petrol based, and then heat it. IT SOLIDIFIES! My good friends dad (knows all sorts of stuff) showed me this and I'll always recall it. I make sure mine is changed with the same brand, because mixing the wrong two motor oils will leave deposits and varnish like you wouldn't believe. Most folks never see the inside of an engine so it never comes to mind. What came in my FJ is Castrol brand, so I'll have to stick with that for now. You know why they make that commercial engine flush stuff? Bingo. You can also use diesel fuel to flush an engine and then run it on idle for a few seconds, rev it a couple times till it sounds dull and then shut off and drain it. You will see big nasty black boogers of stuff coming out, that's normal. Only thing with the diesel fuel trick is you have to run a couple quarts through the engine just to 'wash' it out and then change the filter. Then when you add the new oil just listen till the filter picks that last of it up and top it off. Works awesome on VW flat-four boxer engines and really well on 4 stroke power equipment like lawn mowers. The commercial engine flush is great too but that stuff is caustic as hell and you don't want to get it on your skin.