...do I ever need to do an oil change? It is a 1979 Merc 280S with 2.8L carburettor engine, 6.5qt capacity. >300,000km. Using Shell dino 20w50. That means in 13 weeks I would have topped up 6.5qts of fresh oil.
I'd be doing an engine change!
Seriously though, when it consumes that much you could probably just change the filters on a regular basis and keep topping up with oil as needed. Eventually you'd need to get rid of the contaminents in the oil (such as fuel) so you should do a change at least once a year. At this point the engine probably wouldn't last much longer no matter what you do anyways.
The turbines at work turn over about 20% of the "sump" volume per annum, which appears to be ample make-up to keep the oil state in the fit for service zone for THAT application. RBOTs look good, and the hydraulics are very clean when we strip them.
But we do have a super duper filtration system (sub 2 micron), and a centrifuge that pulls the water out of the system.
So I'd guess that if you got the particles out, that you'd never need a change.
23 years old & still refuses to konk-out...
Look....Scooby Doo spotted in Singapore!
p.s. Use to own a 1975 280S, but I fed it cheap $5 per 4qt noname oil. Kept topping up half qt every week, but engine died after 1 year w/o oil change & filter change. Not exactly died but gave off lots of blue smoke.