The Lotus Grand Tourers

The old girl (Henry's transgender, it seems) hasn't had an oil change in at least 15 years, so I think it might be a good idea to give him (told you) a full service.
I'd quite like to change the oil for new, run it for a very brief period, then drain and put more new in again. I'm talking startup in the shed, move round the yard, to the MOT station and back. Less than 50 miles.
I'm loath to throw away really good top quality stuff, which will be going in at the 2nd change.
Any suggestions for temporary clean-out oil?

I'd suggest not using really nasty stuff for that flush change as you'll leave quite a bit in the system during a change. As well as what clings to the inside surfaces you have a fair bit in the cam towers, head, oil pump and cooler system. Like that I'd suggest something such as Carlube 10w60 or 20w50 as a minimum quality.

just use an oil flush, plenty around, then run with valvoline 20/50 for a bit as its only £25 for 5ltrs

In my opinion - I wouldn't even run the car before flushing it!
Drop the old oil straight away, fill with a 10/40 semi synthetic (about 20 quid). run this up to temperature and then add the engine flush.
Engine flush is only a very very thin oil - something like 0w20 or less to reach areas the thicker oil can't.
Drop this and either run through a litre of the oil you are due to be using for the final fill up before replacing the drain plug to purge the thinner oils.

If you are really anal like me, after dropping the oil with flush - replace the drain plug, fill with new oil to the bottom of the dipstick minimum line, turn engine over and leave idling for a minute before dropping this and filling up with fresh oil. This will take care of the 1.5 to 2 litres of old oil that sit in the head, cam covers, pipework and cooler.

A word of warning though - older engines (typically over 70k that have never been flushed before) may never run quite as well after a flush as the gaps that were once sealed by carbon deposits will have been removed by the flushing. In some cases you are best just using a slightly thinner oil like the 10/40 to push out the old rubbish.

Your engine is only as good as its weakest component and oil is the life blood that runs through it.

Thanks for the replies, all. I'll go with Mr The burn (May I call you Fuel?)
I don't want to break it!
The engine's got 32k on it, so I'm hoping that fully flushing it should be OK.
I know I shouldn't have run it with the old oil in, but I was too excited not to! Next job is the clutch cylinders so fingers crossed.
Then a full service before running it again.

It's Kieran for future reference.
My advice sadly comes from experience of reinstating a Renault 5 turbo 2 after being laid up for 11 years. The oil had lost most of its lubricating properties in that time and I too was too excited to wait and ran it for a bit after spending months re-instating the thing. I managed to strip a liner on a 15k Rallycross engine

I had to run with old oil in my excel for a month as well until I could grind out the old drain plug that had been mashed in by the previous owner. I doubt it did the nicasil liners any favours on a 45k engine but I knew I would be re-building it. The previous owner had obviously run the car for probably 5 years and ignored the oil and just pulled it out for 300 miles each summer, not conducive to longevity.

I won't ever take that chance again - all for the sake of 1 container of oil

32k should be perfectly fine to run a flush through. It's a good time to keep a good low mileage engine in good fettle

Of course, we're talking about the engine as it stands now.

When it's a high-compression 2.5l supercharged engine in the future...

OK, one step at a time.

Now there's a beautiful thought - a 2.5 on a rotrex charger