Did the XK drink buckets of oil when touring and need 1/2 gallon top ups at every fuel stop?
I think you better put up a link to the scan of that handbook.
If there isn't a scan on line you better scan it and stick on shutterfly or some other photo share and then link that.
Or at least quote it verbatim.
Oil has a "normal" running temp of about 100°C. The filler on a car engine is almost always in the rocker cover. It drains fresh oil over the warm top deck of the head water jacket, down drain tubes in the block to the sump. In the sump it meets 3 - 5L of hot oil. 1/2L of
20°C oil added to 3L of oil at 100°C will reduce the sump temp to 88°C. Added to 5L the temp is 93°C. It's hardly going to cause a damaging huge rise in oil pressure or poor oil pickup to pump. Not even if you go racing off from the pits redlining though the gears.Dumping 3L in to a 1/2 full 6L sump or tank in a 24hr race pit stop could do something. Oil temp would drop to 60°C and that is clearly nowhere near up to race temp. Viscosity would rise and hence pressure but flow would reduce.
If you are going to engage in that sort of stuff you have to train the driver to read oil temp and pressure gauges and use the engine accordingly.
None of the above applies to Honda Civics, they can start the engine stone cold and go flying off at 6K rpm with impunity.