C1600 Parts/Service manual.

Something Ive never seen mentioned with the oil angst crew is the problem of modern coolant leaking into engines ......this stuff(green,blue ,yellow,etc) keeps the block and head passges bright and shiny ......but if it leaks into the cylinders and sump ,it seems to be very corrosive ,etches bores with water marks ,and when the glycol mixes with engine oil ,forms a brown sludge that blocks filters very quickly.....This is where the danger lies with old engines ....as these coolants have a very penetrating action on old head gaskets, gaskets that are solid with water or soluble oil ,start to leak with fancy coolants......Beware of freezing ,tho.

That's a bit harsh on the GMs John. They don't burn much oil, they leak most of it.

I used straight 50 (Castrol DD50) in my Inter for ages to cut down on oil usage (halved it over 20/50).
Currently using Penrite 40/70. I can actually go several tanks without a topup now.
Unless the thing has sat for decades, it will have had detergent oil already in it. If cost is more of a factor than oil consumption, the cheapest 20/50 you can buy will still be heaps better than what it got when new.
Do pay attention to John's comment about the zinc, though.

Jarrod.

Most of the suitable oil is now termed "antique vehicle " oil and is around $4 a litre......only a nut ,a rich one, would waste $25 a litre oil on an Inter .......... These engine dont have valve seals ,so use fair bit of oil .......dont get the idea the oil is set and forget between changes ......check the level regularly.