400ml to half a litre per 1000 kms is a little above average but not a reason for undue concern.
May help to use 20w/50 Castrol GTX right up to 20w/70. I have been using "Forte Compression Stabilizer" for the last 20 years as a VACC Senior Mechanic in my Service Centre on clients cars with 90% success while using 15w/40 to 20w/50 standard Castrol oil.
I use it in my own GTV6 and XY351 just out of preventative maintenace; both never use more than 300 ml of oil between services of 5000k, and I always use
Upper Cyl Lube in
every tank of fuel since I started driving over 39 years ago which has proven to avoid premature wear on the Pistons, Rings and Cylinders !
The previous owner of my Capri did the same thanks God; proven by my recent internal examination of my GTV6 soon after I purchased it with a burnt valve. All aforesaid parts were like new !
None of my 12 vehicles ever go longer than 5000k between Oil&Filter changes, except the Prado which uses 100% synthetic for 10,000k, mainly for fuel economy, which I wouldn't believe until i saw it myself!
Good luck
Bernie