On my 82 280zx n/a I switched to synthetic at 168,000 and I now have 219,256.2 and there is no knocking or oil leaks other than I forgot to tighten the back 2 bolts on the valve cover, but it does not drop an oz. of oil. forgot I use mobil 1 synthetic 10-30W

 

Joe, first off, welcome to the board, bro. Now that I have used three comma's in an eight word sentence... I just bought an L28ET, but am still running my L28E. The L28E has about 3,000 miles less than yours, and I have been using some synthetic 20W50. It's been fine. No REAL problems, of course driving an engine with 268,000 miles is bound to have some problems. unless you are REALLY lucky. Later

 

At your next oil change run a good motor flush to loosen the rings and other gunk, follow the directions exactly. Then run Moble 1 20-50. Repeat after 2000 miles and your engine will last a lot longer.

 

This is an issue that has been debated at length by some seriously knowledgeable people, one of whom I know is a petroleum engineer.

Leaking has been known to happen with older high mileage cars switching from petroleum to synthetic.

Once the seals and gaskets have aged and dried up to an extent, the petroleum oils and their 'gunk' can actually help them retain their sealing function, whereas the smaller particles in the synthetic can lead to removal of the gunk that is helping the seals and leech into those tiny little nooks and crannies that the petroleum oils couldn't get too.

Why it effects seals the way it does may just be the downside of decreased friction at the critical point of sealing (at the sealing lip).

New seals have a sharp defined sealing lip (or two sometimes) but old seals tend to wear "flat" and lose their static tension on the shaft or area they are designed to seal.

The change in the shape of the seal lip(s) is known, though to what extent this change is effected by synthetic lubes is still conjecture at this point.

The bottom line is that the manufacturers of this product make no mention of such a problem on the box itself, so either they aren't aware of the problem, or don't choose to believe that one exists.

So, the answer to your question is, it may work fine or it may not.

If you do decide to take the dive into synthetics and you live in an area with a relatively mild climate, use the 20w50. Hopefully, this will be heavy enough to avoid the leaking that has been reported while still giving you the advantages of a synthetic oil.

If you live in a cold climate at least some time during the year, 20w50 is going to be pretty slow to make it's way throughout the engine during a cold start. At that point, I would avoid the synthetics and go with a good quality dino oil.

Cheers!