NEVER use synthetic oil (or any oil additives) for break in, it's too
slippery for the rings to bed in properly.  Most sources I've read like
single weight dino oil, either 30w HD or 40w HD, for the first 3000
miles.  Change the filter and oil after the first 500 miles.

Don't squirt anything into the cylinders either.  Some rings are meant
to be installed dry, others are lubricated with oil upon assembly.
Rings need friction to hone themselves into the cylinders. Do remove the
plugs and crank the engine for 30 seconds or so to build up oil
pressure, then put them back in and fire it up.

It is important that the engine fire up right away or you risk damaging
the cam with too much cranking or short bursts of running and then
dying.  Once it starts, run the engine at 2000 rpm for 20 minutes (no
idling), this lets the cam and lifters bed in as well.  Watch your oil
pressure and temp gauge, and look for leaks.   Then you can drive
normally, but good old around town driving with lots of speed changes is
best, avoid constant speed highway cruising and serious stop and go
traffic (you don't want to do too much idling) for the first 500 miles.

For those who what the rationale behind all of this:

The theory on the 2000 rpm for 20 minutes, as well as avoiding idling
for the first 500 miles, is that at low speeds the lifters are pressed
tightly against the entire cam lobe, which can cause galling.  At higher
speeds the lifter is accellerated rapidly up the cam lobe and is nearly
weightless at the top, which is where damage can occur during break in.

The reason for avoiding constant speed during break in has to do with
lubricating the cylinder walls.  At constant speed the cylinder oiling
may not be adequate for the higher friction levels during break in.
When you accelerate the engine compression pushes the rings into the
cylinder walls so they can bed in, and on deceleration the vacuum
increases and pulls extra oil up into the cylinder for lubrication.
Constantly alternating the two provides the best possible operating
condition for proper ring seating.

I've used this method successfully on at least 8 engines over the years.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT