need to know what type of oil to by for the six....aprox how many litres

Which 6? 3.0?

~9 liters says the spec book and owners manual.
assuming stock oil tank, no external cooler, etc...
assuming 3,0 or less - but if it were bigger, you'd probably have an external cooler...

20-50 is fine, especially for dino oil and a summer car.
i like 15W-50 synthetic. but the engine is pretty flexible in this regard.

too much oil is bad - it'll just puke out the tank breather and make a mess, or get sucked into the intake and make a mess -and- an oil cloud ...

it is a 3.0 six with a 914-6 tank, and a remote oil cooler up front

The weight should be fine.

I've heard another 4 qts for the cooler and lines. CHECK THOUGH.

I get 10 qts out at an oil change. I initially put in 12 qts.
I use 10-40 plankton.

Mine runs on 20-50 and seems to do well. It's a 2.7 with no external oil cooler and runs around 180 to 200 oil temp.

The hottest it ever got was 210 over the grapevine and then cooled right back to 180 after I crested the top.