What oil.... semi or Fully sythetic

[Jonny Boaterboy]

I have been wondering about what oil to put in my 3.9 V8 that has done 153,000 miles. I allways try and put the best in were I can. I was using morris semi synthetic 10-40w but at the last oil change switched to Morris fully synthetic 5-40w I have now developed a rattle on the engine when it is put under load..... not sure if it was there before I changed to the fully Synthetic but it's there now. I have been told to switch back to a semi synthetic 10-40w as my engine is a bit long in the tooth and the extra thickness will quite it down again. In my hand book it says you can use 5-40w or 10-40w So which do you go for? Fully synthetic keeps your engine cleaner but semi might quite it down a bit?

I'm not sure what the rattle is exactly it sounds like it is coming from the right hand side of the engine only and I don't think its the manifold blowing.

I am the only person that ponders on oils or does anyone else look into it as much as I do?

Thanks all

Jonny

[andyb]

I second the 'stick with semi' argument. I have know a number of high mileage engine killed because the owner put in a 'better' oil.

[Jonny Boaterboy]

Ummm maybe I'll switch back then! the whole point of puting better oil in was to preseve the engine not kill it!

Thanks Andy

[tack43]

How about, use a cheaper oil (such as castrol gtx) but change it more often - every 3000 miles

[Eeyore]

I am the only person that ponders on oils or does anyone else look into it as much as I do?

Nope, far from it. Oil is one of the most important components in the engine and one that sooo many folk overlook. I reackon a good number of the mysterious v8 misfiring problems are actually oil related (especially as some poor folk change everything electrical and the dang still misses)

My choice would be a good quality synthetic, but of a slightly thicker consistancy (like the 10-40). If memory serves this is what was used in our racer. Good oil won't foam as readily as cheaper stuff which can't half make a difference. It does however sound like you've got the ol' v8 rattles - this normally indicates wear in the cam train, so it may also be worth thinking about having a look at the state of that at some point.

Cheers
Eeyore