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Straight sae30 or multigrade, and mineral or synthetic ?

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Up to now I've been following the VP manual's oil recommendations for my AQ205A petrol engine ( 1989 4.3 litre V6 GM with carburettor) .

This states mineral, SF (SE) 10W/30 in the official operator's manual, and SG 20W/50 or 15W/50 in the official workshop manual.

I've been trawling the web (big mistake) and there's masses of contradictory opinion out there, especially the USA, where these engines are much more common. Trying to summarize my thoughts so far:

I was tempted to go fully synthetic, because it "lasts longer" and "doesn't break down", but I've read in a number of places that it's a bad idea to switch to synthetic from mineral after a number of years because it can cause seals to break down ?

More confusing is the advice on whether to use a multi-grade or a "straight 30", which many people seem to favour instead.

There's also much comment that a 30 year old design cast iron vee engine, based on a GM commercial truck is far less demanding than a modern turbo, and "almost any oil will do", and that the regularity of changing it is far more important instead.

I've stayed so far with the mineral 10/30 because of the fear of damaging the seals, and because I do use the boat quite a bit in the winter, thinking that the lower viscosity at low temperatures was important.

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Thanks for the advice Setfair, I'll give him a try.

Strange thing about this thread, it doesn't seem to appear in the "view active topics" search at all.

I can only see it via the individual board indexes ?

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