I just spotted this;A flushing oil (not additive) as available under Comma nd Granville brands. It is a high detergency mineral oil so should be able to be used for a few miles or idle/fast idle for 20 minutes. I have used both on various engines (TS & JTS) and not had ill effects.
Remember to allow the old oil to drain out for ages though. If done right, all of the 5 litre can of new oil should take it to maximum. The only downside is the oil level is harder to read because it is and stays so clean.
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I guess after draining , if it's not that possible re-assemble and run the car with a cheap oil then perhaps it would be prudent to put about a litre of oil through to push out the old oil and flush?..
I've got my mechanic to do oil changes on my car as it's previously been lowered and there really is not the elbow or nose room to even get under there!