Winter is around the corner we get to -40 farenheit some days ,I was thinking about going with Lucas 75w140 full synthetic instead of 85w90.the front diff is a M60/2 and rear M70/26 with anti spin.

A full synthetic 75w90 would be the best choice overall.:thumbup2:Just make sure to add the friction modifier to the rear diff oil or you will get chattering or popping in corners with the clutch type limited slip. It hurts nothing but can be annoying to some.

On the gallon of the Lucas 75w140,it shows you don't need the friction modifier.

Don't always believe what you read. The clutch type limited slips do not like full synthetics without friction modifier. I had this go round years back and talked with both eaton and amsoil tech departments back and forth and it came down to the fact that the clutch disc manufacturers have not kept up with the rest of the industry in regaurds to designing clutch material that works well with full synthetic oil without the need for additives and even some syn oils even with modifiers will still cause thew discs to chatter. For example the amsoil severe gear brand lubes always resulted in chatter even with double dose of modifier and the regular amsoil gear lubes did not, it all comes down to the additives in the oil.I would still not reccomend a 140 oil in the North country unless it pulls 20K+ loads all day everyday.

I have a 70 rear with factory posi and I put valvoline 75-140 in it 15,000 miles ago with no issues to this day, I believe I read in the owners manual or the factory service manual that allowed 75-140 if you wanted to run synthetic, however it was not required.

I heard from another guy that 75W140 was a good choice as he didn't have any problem when he switched over,he didn't have to add a friction modifier.

  wagntagn said:

I heard from another guy that 75W140 was a good choice as he didn't have any problem when he switched over,he didn't have to add a friction modifier.

I haven't added a friction modifier.

It isn't going to hurt anything without it like I said you may develop a chatter while cornering. 75w140 isn't going to hurt a thing at all either just saying its just overkill for your needs.If its what tickles your fancy and pocket book just go with it, sounds like you already had your mind set on it.