A friend has an MGC and is changing the oil, it takes an awful lot, about 7 litres. He has put in Duckhams classic 20/50 and wonders if its ok to mix it with Silkolene Silkolube 20/50, I think it will be OK. The Duckhams claims to have a zinc additive, any ideas?

Usually though best to not mix oils. But I can't see an issue, they are old fashioned dino oil, very basic.
The Duckhams additive will be ZDDP, supposed to be needed for flat tappet engines. It is probably in teh sikolene stuff too. E|ven the fully synthetic stuff I use in the st170 engine has 1000ppm of the stuff....

Here are some replies from the MG Experience MGC forum to a question as to what engine oil to use in a MGC;
"No dilemna.
The ZDDP is essential in cam brake-in of flat tappet engines. Once that has happened, I personally believe that its importance is somewhat diminished.
Can't go wrong with good ole Castrol 20x50....add some additional ZDDP (available from STP) if you wish.
Mobil One makes a good synthetic 20w50....but knowing you....your oil changes will be more frequent than the life of the oil.
Stick with dinosaurs".

"Using Valvoline 20w50 VR1 Racing Oil, not the synthetic racing oil. High in ZDDP."

"Agree with all above. Use a good oil, change it every 3000 mi and add a dash of zddp if desired. I like Castrol but will buy wnat is on sale as long as it is 20w50. I dont need synthetic."

Hope this helps

Thanks for the replies!