I have heard many people give conflicting opinions about using using synthetic vs "fossil" oil in Z cars. Is it true that seals will leak on an L24, or any engine for that matter, if synthetic oil has never been used? I am debating about using synthetic or not in my 73 240Z. What would be the best weight to use in both synthetic and conventional oil. Also, I heard that you can use synthetic motor oil in the diff as well. Any thoughts?

Brad

This is just my experience, but I'm running 20-50w Valvoline synthetic in my Z (engine is stock) and haven't had any problems at all with it like seals leaking or anything like that.

20-50w is kind of thick IMO. I've always run 10-30w and though I'm not a fan of synthetic, I never heard or seen any substantiatable proof of it causing leaks. My take on that is, these engines are old and if it started leaking after putting synthetic in it, it was just it's time.

You will find a LOT of opinions, but if you do a simple search for "synthetic" you will get 4 pages and 100 threads. Good luck.

Thanks for the input. I guess I will continue using fossil oil b/c that's what the engine has had all these years. I change the oil every three months or 3000 miles whichever comes first. I prefer Castrol GTX 10w40 or 20w50 depending on the season.

Synthetics don't directly cause leaks. But they are more detergent than fossil oil, and so they can clean up sludge and varnish that may actually be what is doing the sealing. So an engine may leak after changing to synthetic, but the only fault of the oil is that it is cleaning too well. But it takes the blame anyway.

I use synthetics in most of my vehicles, but will probably stay with fossil for the Z. But if I had a freshly broken in L2x that I knew was clean inside, I'd probably do synthetic there as well.

Thanks Arne for the input. You guys have forgotten more than I will know about Z's.

Brad

I'm with ya on the Castrol GTX. I run it in my 240. If you are worried about your seals, on your next change try running one quart low and adding a quart of Lucas Oil treatment.

Guys,

I used to live in San Diego, Ca. and drove a 240. I lived in La Mesa and to get home I had to drive up the I5 past SD State U. The hill was a long one and I usually ran the couple miles in 5 th at 80 mph. In the summer heat of 90 degrees this properly heated my oil. At my off ramp I would "dive" off the freeway and come to a prompt halt at the end of the off ramp. Penz 40 wt would let me keep an indicated 10 psi of oil pressure at that offramp. I used that spot to check the viscosity of my oils. I tried them all and Cast 20-50 went to dead zero psi at that off ramp. Only Penz let me keep even a skosh of oil pressure and I kept her topped with 40 in the summer and 30 wt in the winter if i stayed in town. If I went skiing I filled with 20 w 30 wt and that was too thick in the 20 degree mtns. but made due cause the trip up was at 100 mph and I had to have somthing to keep the metal appart.

I now have a Lexus SC400 and it uses 5 wt oil and I can't believe it but I follow the instructions. I don't think my 240m would have stayed together with this oil.

Thats my story and I'm sticken to it.

John